I think it's because black women for the most part are taught to be a mental power house by their mothers. The Black family has been broken down for almost over a century so most Black American children grow up without fathers. So the girls are taught that "Black men ain't no good!" and are taught to be strong and independent.

Stereotypically White women are told to be supportive and submissive to their husbands so I guess a lot of White men aren't used to the "go-getter" attitude of Black women.

That's my opinion anyway. It's an interesting subject.