Family pressure... because being a divorcee is shameful in some families and cultures.
Sometimes the good parent is so ground down by the other that they can't imagine life without them. Their confidence gets eroded over time, and sometimes, the partner gains some form of control over them.
Fear of what they don't know, e.g. if they can cope alone, if they can manage financially, where they will live, if the other parent will look after the children when it's their turn, etc.
We get told that it is important for children to have two parents. Actually, one decent one can do a better job than if shackled to an inadequate/violent one.
Society's judgement can be harsh. It used to amaze me how many people looked down on single parent families, when in my experience, it was so much better than what our unfortunate norm was before. I had a teacher who informed my class that single-parent families lived in rented accommodation, in terraced housing in a notoriously poor area, with sons who went to prison and daughters who got pregnant in their teens. Someone piped up, "TheeNotoriousPIG doesn't!" I was the only one from a single-parent family in my class, and I shot down all of the teacher's accusations in one go, because I lived in a detached house in an expensive area of a nice village, it was mortgage-free, my brother was at university (and had a clean criminal record), and as I'd hit 17 without a boyfriend, I was unlikely to have a baby any time soon. Also, I could read music. Apparently, children of single parents couldn't do that, because they can't afford music lessons...
We were lucky in that our so-called dad ("A wonderful man" in public, and abusive behind closed doors) died, because I think that my mother (a good parent, but ground down) would have stayed with him due to a lot of the above reasons. We were much happier without him. This was in the 2000's...