well, that's a million dollar question, isn't it? here's my take:

mostly, these women wind up with assholes because they confuse abusive behavior with strength--this is a guy who can "take care of business." and when a guy like this treats them badly, the main two thought processes are (1) it's really my fault anyway, i shouldn't have done X, and (2) maybe if i'm good enough, nice enough, whatever, he won't want to treat me this way. the common denominator is these women see the guy's behavior as a result of something *they* did.

ok, so you wonder why they bypass all the guys who were nice to them. well, one reason is that they figure *everyone* is going to be an [censored] to them eventually, so at least the guy who's an [censored] from the beginning is laying his cards right on the table. the guys who's nice is more scary because you wonder what he's hiding. (obviously, the rational answer is that he's probably not hiding anything--he just wants to be nice to you because he likes you--but if your parents and other relatives kept telling you how inferior and worthless you are your entire childhood, you're going to be suspicious of *anyone* who thinks you're worth anything.)

actually, i have a little more sympathy for a woman without kids who stays with an abusive guy than a woman who has kids. once you have kids, your primary responsibilty is their safety and welfare. so staying in an abusive relationship means you're neglecting your responsibility to your kids--you're effectively telling them that they aren't valuable to you (or at least not as valuable as the abusive [censored]), because if they were you wouldn't make them live with abuse. but that's just my $0.02.