For the same reason sexual abuse goes on today, and not necessarily black women. A women is intimidated by her preditor who she believes has some sort of power over her (be it her slave master, husband, boss, landlord, even drug dealer). The end result is the sad reality called sexual abuse and it happens all over the world. But to the root of your question, black women feared white men. They were born and taught to fear them. So just because slavery was abolished, do you seriously think that they would no longer fear the men they saw rape their mothers, kill their fathers and brothers, and do it just because of the color of their skin?