Because they are taught to be "strong black women" and most of the time, their definition of "strong" is to be a bitch. But also, black women know how to get what they want. My mom does the same thing. When someone at the store fucks up, she calls them on it and makes a scene. It's embarrassing but, hey, at least she isn't passive.
Also, black American culture is matriarchal in general. The source and pride of the family is always the grandmother. Always. If not that, the mother. So in many cases, she tries to take the job as both the mom and dad, even if there's a father in the house already.
But it depends on the woman. I can think of just as many passive, quiet black girls as I can think of ghetto black girls. The passive ones are very often from the ghetto, the dominating ones are very often from the suburbs.
Like all things in life, shit is too complex to pinpoint one sole reason.
