When abused fear for themselves, children, and family will be a large reason to why women feel they need to stay in these abusive relationships, of any kind, not just physical. Of course many factors can play a part but I believe fear is a major reason as to why women stay.
There could also be a sort of stockholm syndrome where the captor or the abused member starts to support the aggressor and sees what is going on as their own fault. Also I would like to note that not only women are abused in relationships and that a lot of time men are too which is a point missed by a lot of people. Also psychologically people want what they cannot have due to supply and demand and desire. So while fear is common and is a real concern for a lot of abuse victims I would not say that it is the only big reason for staying in the relationship.