It's all about patterns and comfort levels. What you need to realize is that people, not just women, are most comfortable when they are in a pattern of behavior that they recognize and are comfortable with.
When it comes to relationships this can be a dastardly and even deadly combination. If a woman grows up in an abusive (mental, emotion, physical, sexual) situation she is going to recognize subconsciously that type of behavior as "proper" for intimate relationships.
It's why women who were abused as children mostly end up in abusive marriages or relationships.
So all this said, the reason women don't leave abusive relationships is because they feel that that is what love really is. Even if on an intellectual level they know they should leave, their guts literally tell them, that this is what a relationship is supposed to be like.
It takes an extremely strong and insightful woman to break that cycle.
And by the way, don't think that just because you are a "nice" guy and her "best friend" who she says has tons to offer to the "right" girl means that she is interested in you. She's not. She is damaged goods. And she needs professional help to break that cycle. Your offer as good as it might seem doesn't fit her current ideas of an intimate, sexual relationship.