Maybe they don't approve of the person that their child picked?  Maybe there was another person they preferred specifically.. or just generally would have preferred due to cultural... socioeconomic.. or other reasons?

Maybe they don't approve of their child's life and blame the new partner?

Maybe they always preferred another sibling..

Maybe they love their baby so much that no one would be good enough.

Maybe they are insecure

Maybe they are emotionally unavailable people?

Maybe the new partner is a problem themselves and got of on the wrong foot?

My inlaws are a HUGE conglomeration and the self-Family love is so HUGE that its a THING.

self-absorbed so much so no one else exists.

My SO told me a story when we started dating. It was that when his cousin's mom found out he was dating a girl not from their culture (they are immigrants to the US) she started crying. He said that's how all the moms are of that generation in his family.

I thought, surely i can win them over. I asked him what i could do that would make them like me more. He told me that when we go to their house, help clean up. Another friend told me to always take off my shoes. I swept, mopped, wiped, washed, all in no shoes. I brought homemade dessert to family holiday gatherings. But, the best i ever got was polite tolerance.

It also doesn't help that SO's oldest brother, who is looked at as above everyone else because of being older and male, still has a weird attachment to BM2. They hang out and do holidays and birthdays together. When he went to their home country to get a wife, as did SO's only other brother, he introduced the new wife to SO's BM2 first thing, even though SO and BM2 had been divorced for 5 years at that point, so now the "top" SIL is best friends with BM2. The "bottom" SIL is just trying to survive day by day and doesn't take a side but she doesn't speak any English so there's not much to be done there.

I tried really hard to integrate into the family for about 3 years. I took lessons in their language but they speak a different dialect from what's offered and it's a tonal language, so hard. BM2 was still on their family text chat (which is in English) so SO tried to start another one with me on it and her off. But someone somewhere just remade it without me. SO says BM2 isn't on it, but idk. And at this point, i just keep a polite distance. I'll go to Christmas and such, still take my shoes off and help clean. Still bring a pie. But i don't go out of my way and i minimize the times i go. It is what it is and i don't stress anymore. 

My DH and I have grown, married kids and grown, married grandkids.  I've noticed a weird thing about us.  We acknowledge and welcome the first spouse with open arms.  Time goes by, maybe some babies arrive, maybe not.  Then they divorce.  After awhile, they introduce us to #2.  I find that altho we are cordial, we aren't as close as we were with #1.  In each case, the #2s have been fine people, no complaints.  I cant explain it. It's not like we maintain contact with the #1s altho we stay polite due to ongoing grandkid events.  Just reporting from the inlaw side here.....

Lack of confidence, inferiority complex, failed life performance, though more likely sub par character, no honor, and inability to recogize the shit that they are.

They being toxic ILs.  Whoever they may be.

My XILs were interestingly the prototype of this.  On the surface a quality family, strong in their church, strong in their community (XFIL was on the town council). They lived in the it zipcode in the metro area, their home was in an elite enclave in that zipcode which was the desirable suburb.  They owned a 200acre ranch an hour N of the city.   None of that made much sense considering that XMIL was a book keeper and XFIL was a low level government rural property inspector for the State.  The first example of their issues was when their eldest, their only son, married a woman that XMIL held in significant disdain due to her family background.  A rural low income family background from a swamp communty in a neighboring state.  XMIL was so anti DIL that her own son eventually took his family and left the state.  Good thing he did.  The why of him taking his family and leaving being a good thing will become clear below.

My XW turned out to be a serially adulterous skank whore.  The marriage lasted 2.5 years from wedding to final divorce decree.

I moved on, met my incredible bride of 30 years, we raised a man of character, honor, and standing in his adult life, profession, and communty together.

XW is now on at least marriage #3, I was DH #1, with three all out of wedlock spawn. Two are cheat babies conceived with a cheat partner while married to someone else. She was knocked up with her eldest during out divorce, and got knocked up with #3, another cheat baby, while married to the geriatric fortune 500 executive sugar/baby daddy she left me for after he finally married her following the birth of their second.  Blessedly I escaped without polluting my gene pool with her.

XMIL turned out to be a federal felon, convict, and prison inmate who embezzled $Millions from her employer.  Which explains a lot that was perplexing during my dating, engagement, and blessedly short marriage to their eldest DD.  They pretty much lost everthing.  Her employer sued the whole family, except for XBIL who was long gone, and won a $MultiMillion suit against them.  My XW was on the hook for $Several Hundred K of the payback according to update from mutual friends.  XBIL escaped a couple of decades before the Federal Marshals arrested my XMIL as the family left the final hearing of the civil suit.  Though they they had to pay ther former employer $Millions, she ripped that business off for millions more than the family had to repay.

My current ILs are not without their challenges. Low income laborers with highly developed unfounded superiority complexes.  Particularly two of my DW's three younger sibs.  BIL2 has a heart of gold.  MIL is a gem.  FIL is deceased.  He was a bit more like two of the youngest and never really understood what his eldest DD and I have accomplished in our life together and our careers.  MIL really does not comprehend her eldest's success either but she is a good natured loving person.  Even with the major differences, DW was FILs person. He was far more closely connected to her than her three yunger sibs.  Even MIL has stated that DW was FILs person.  DW was FILs golden child though he did adopt her. DW's biodad/MIL's first DH, was killed in an automobile accident a few days before my MIL found out she was expecting with my DW.  FIL was at the hospital when DW was born. He and MIL married when DW was 2mos old. They had been raised together and their families were BFFs while they were growing up.

My DW is the mutant in her family.  A highly successful graduate degreed professional. Her family generally considers her the naive rich city dweller who does not know how to function in the world.  Interestingly, they seek and take her advice on financial decisions, taxes, etc... but .... are raising their own kids to replicate the multigenerational struggle to live beyond each successive paycheck and embracing instant gratification that they just cannot afford.  They are all, all about the dream with no concept of living within their means.

The ILs have a common familial trait of an unsubstantiated superiority complex that limits their work advancement,  demonizes anyone and everyone who is not them, and turns them collectively into a fairly insular entity in their small town community.

It is kinda sad.

But, they are not bad people.  Though they do infuriate me to no end with their self created drama and their breaking of my incredible brides heart.

My family, from my perspective, are low drama and fully accepting of my DW.  They were hesitant with my XW due to the things that made them go Hmmmmm? regarding my XILs.  As it turned out, my parents were absolutely right about my XILs.

My family absolutely worships and accepts my DW as their DD, sister, aunt. DW is truly that special. She loves my family to death.  They also accept SS as theirs.  Their eldest GK, their only Rags clan nephew and cousin. Not too hard considering we started out blended family adventure when we married the week before SS-32 turned 2yo.  Our son adores his GPs, his uncle and aunt, and his cousins on my side of his life.

Overall, I believe that DW and I fairly well won the IL lottery this time around. The SpermClan, pretty much is made up of an odor emanating shallow and polluted gene pool that we thanfully rescued our son from having to splash around in beyond the expiration of the CO we lived under for 16+ years.

In-laws -- the relationships can be strange.

My first mother-in-law was an immigrant. She moved from her home country to an enclave of immigrants from the same country. She understood English, could read it, but her spoken English was limited as she never really had to use it in the community she resided. I was the non speaking home language daughter-in-law she never wanted. But, she tolerated me. DH1's sisters followed their mother's lead. The sisters and other sister-in-laws would get together and I would not be invited. 
Years passed. MIL died. DH1 died. Somehow over the next years my relationship with the in-laws got better. They started trying to have a relationship with my kids (and I encouraged it). I remarried and they love DH2 and invite both of us to family events. 

Second marriage -- the in-laws hated the deceased ex wife. So any relationship I have with them is better than hers. I'd say our relationship is cordial and that's where I'd like it to stay.

My MIL most definitely prefers BM to me, even though BM cheated on DH multiple times and then left him. I've been married to DH longer than he and BM were together, so longevity doesn't explain it.  Lucky for me, MIL lives hundreds of miles away from us and I very rarely see her. I used to make a lot of effort with her, but she has treated me poorly so now I don't bother.  She's still loving every social media post BM makes while I, OTOH, have her blocked.  MIL also prefers my SDs (and also BIL's kids) to DD, but at this point she's pretty irrelevant to DD, and my parents are fantastic and present grandparents to DD, so I don't think DD is missing much.  I sometimes wonder why I was treated as the 2nd class daughter-in-law, but ultimately MIL is the loser in this situation, as her relationship with her son has suffered as a result and she has almost no relationship with her youngest grandchild, who is a great kid.

Are in charge of your life.  You control where your life is going.  If your in laws disrespect you, or show favoritism to the ex.  That's up to them. I just would have nothing to do with them.  They will get the message sooner or later or not.  This is not up to you 

Came back to say it's like going into a job interview. You dress your best, ace all the questions, know you are very qualified. But everyone else at the table knows there's already someone selected for the job. They have to hold interviews for appearances, but you have no shot. You may even go back for second or third interviews, take off work from your current job. But you are wasting your time. 

My dad was killed in WW2 when Mom was 3 months pregnant with me.  A terrible time for all, I'm sure. Mom stayed close with his parents and I was very close to them, especially my grandmother.  Flash forward a few years and Mom remarried.  V was a good man, devoted to her, wanted to adopt me.  On the negative side, he had no experience with kids and was very grumpy.  

They married and as I look back on it, I realize my grandmother gave a master class on how to treat an in-law.  V wasnt even her in-law but she treated him like a prince, cooked his favorite food, laughed at his jokes, etc.  They lived out of town so being able to see me meant making his trip pleasant so that he'd continue to make those 3-hour trips.  Or, maybe it was worth it to him to get me out of the house for a few weeks.  Lol.  

It wasn't only making happy with him.  She also remembeted their anniversary each year and when the younger children came, she treated them like grandkids, too.  Actually, on each of our birthdays, the others would receive something, too, just so nobody would get their feelings hurt.

In her last years, she was in a nursing home and got a little more frank one day.  She had disapproved of the way he corrected kids at the dinner table.  Who would have ever known.

It's been a great example in how to treat in-laws, especially in an emotion-filled scenario.  I wish I could live up to it.