I worked with a victim who was repeatedly hospitalized for the abuse, but she told no one because every time she went into the hospital he would remind her that he had the kids. She was scared to death that if she tried to escape, or told anyone, he would kill not only her, but them (and believe me, she had good reason to think that). She had taken the kids once and run, but he called the older son on his cell phone, tricked him into disclosing their location, and snatched the boy. She came back with her girls because she feared for her son.
Now, I'm not saying that was your mom's experience exactly, but there was probably some version of that going on, in which she felt her best bet to keep everyone safe was to try to keep him placated. When you're constantly beaten down, threatened, belittled, made to feel stupid and incompetent, it can be very difficult to see a way out that isn't more dangerous than your present situation.