As you can imagine, I wonder that every day and have been asked that question point-blank from my SD. I can say that in our situation, there seems to be nothing obvious or insurmountable. From what we know, she's not an addict and while mom could certainly use some psychiatric help, she's not mentally ill a way that would incapacitate her. She is not incapable of working, has no major health issues, has a roof over her head, a working vehicle, and if any of those things were issues, has people in her life like her own family - heck even us! - to help. I guess that's what kills me - yes she's had some challenges, but she has also had plenty of resources but just reached a point of giving up. I don't get it. I would crawl through fire on bloody stumps to be with my children. I don't understand how a mother - who raised her child(ren) for years and years - doesn't have or can turn off that instinct. Even if I were, God forbid, in some kind of situation where my kids would be better cared for by someone else, I would at least be in contact. I would go out of my mind wondering how my children are and would need to hear their voices and see them on a regular basis.
Those conversations are the most painful. All I can tell my SD is that I honestly don't know what is going on, I don't understand her mother either and as a mother I would never make the same choices but right now, the situation is what it is and I pray that someday they have a good relationship again and can heal, and that her dad and I are here for her for life and no matter what, we'll always be here for all of our children. Our family therapist has also introduced the idea of guilt to SD - that perhaps her mom was in a bad place with the family breaking up and getting out of domestic violence situation and now feels guilty about exposing her to that, leaving, falling out of contact and now it's been so long and awkward that the guilt is making her stay away. It sounds plausible to me.