There's hundreds of reasons women or men don't leave. Worried about supporting themselves financially, worried about supporting children, worried about what the family/church will think- especially if they were raised to believe that divorce is unacceptable. They are "comfortable" even if they aren't exactly happy- and they know what to expect. Not knowing can be VERY scary.
And women being abused: oh, there are as many reasons as there are stars in the sky to why they stay. Men don't start out abusive- they start out sweet and loving and they pick women who have low self-esteem most of the time. They shower her with gifts and love and compliments, and she, for once, feels beautiful and loved and worthy. He's jealous, and she finds it kind of endearing- he doesn't want anyone else to have her! But time goes on, they argue, and maybe he slaps her. But immediately, he's so sorry, he'll never do it again, he doesn't know what got into him, and she is scared, but she loves him and knows how GREAT he CAN be, so she stays. The problem is, every violent episode raises the bar for the next, and they will progressively get worse, usually until she leaves or until he kills her. But you have to understand, that these women love these men, despite all of this, because they think back to what it was like in the beginning, and many hold on to that- maybe he'll change! He says he will!
Then comes money, finding a place to live, scared for kids- what if he does something to them? A woman's risk of being killed escalates DRAMATICALLY once she leaves her abusive partner. So, sometimes it's actually safer to stay, as twisted as that is. They could go stay with family- but what if he finds them and hurts their family too? Many women don't want to chance that either.
Point of story- there are always SO many variables to each woman's story. They can't always just leave. It's never that black and white.