Sarahlynn's posts are moving and powerful, and there's not much I can add to them. But that's never stopped me before.
Her first reason why an abused woman might not leave her abuser is, IMO, one of the most powerful and important ones, and the one that most people understand the least:
Abusers are often very smart, very talented, very convincing. They might seem like wonderful men to family and friends. They might seem very honestly apologetic after the fact. And many of us took Psych 101. We know that we respond very well to inconsistent systems of reward and punishment. We love gambling. We prefer stocks to bonds.
Yet in the present case we have a man who, though he beats his wife, is often very charismatic to the rest of the world, and perhaps to his kids. And even if he beats his kids as well, it is known that intermittent affection can be a stronger binding agent than consistent affection. We also have a man who has demonstrated his power over another human being through brutality.
I have a personal philosophy (learned through years of experience): The more charming a man is, the farther away a woman should stay. Because here's the thing -- an abuser does not start using abusive behavior at the beginning of a relationship -- it can take months, often even years, before he starts abusing. Abusers often have a history of being abusive. In some cases, they may genuinely feel that they are not going to be abusive this time. Regardless, they know the possibility of abuse is there, so they overcompensate by being overwhelmingly charming -- to the woman in a new relationship and to outsiders.
In addition, no abuser is abusive all of the time. Even after abuse has happened, there are often times when an abuser is quite apologetic, caring, and loving. In the cycle of abuse, this is referred to as the "honeymoon period." During this stage, and abuser is often remorseful, making promises that the abuse will not happen again, often volunteering to get help in changing, making heartfelt declarations of love.
During these times, a woman genuinely feels that she is loved (and, truth be told, she probably is -- although it's not a healthy love). She genuinely wants to believe his promises, because she doesn't want to have to leave the man she loves. And when he's in this stage, he really can show her how much he loves her.
And, she doesn't want to be alone: all people in our society, but especially women, are socialized to believe that there is something wrong with being alone. People (especially women) fear that if they leave, they may never find another person to be with; that this is as good as it gets. The bad times may be bad, but at least she has the good times to go along with it; at least she's not alone.
When talking with the women staying in the shelter I work in, I strongly discourage them from calling their abusers. Part of that is the whole confidentiality thing; but frankly, the main reason is because it is one of the primary reasons a woman will return to her abuser. After a couple of weeks, the memory of the abuse has faded somewhat (just as the memory of the pain of childbirth fades). The abuser is often desperate to regain control of her, and will do anything to get her back. The promises of change and the declarations of love combined with the faded memories of abuse, the love she feels for him, and her fear of being alone are oftentimes too powerful and undermine her resolve to leave him. This is one of the main reasons that it takes a woman, on average, 4 - 7 times to leave her abuser for good.
Related to this issue is guilt. The guilt women feel due to their being socialized to take on the burdens of all of their loved ones. Last week, I spent a good portion of my week encouraging a woman not to go back to her abuser, of letting her know that the contract she was going to make him sign making him go to batterer intervention and "really working on the issue" was not going to be enforceable when he's choking her the next time. When he tried to apologize, she was able (after talking to me and other staff people) to see through it. When he tried to diminish the harm he had done, she was able to see through it. But what she couldn't get past was her own guilt. He was having to take time off of work (as a bank manager) because of "this situation." He was feeling "bad" about being a "wife beater." They had just moved here not too long ago, and he was struggling with pressure from work, and this was making everything so much worse for him. She couldn't undo that socialization that she was the one who needed to take care of him, despite the fact that he was the one who put himself into this situation.
Love is a powerful emotion. And it can be used as a powerful weapon. Until we, as a society and as individuals, recognize this, we will not be able to stop the cycle of abuse.