Ah good, a linguistic question.
Enslaved Africans overwhelmingly picked up their English in the South, from their 'owners'. That would have been the model they had to emulate.
Having learned and transmitted that language to their own families and spheres, it becomes that culture's dialect.
By the time Emancipation hits centuries later, and by the time industrialization brings the Great Migration to the North and Midwest, that dialect is well-established. For centuries.
Just as any other regional dialect of New Hamster or Texas or Appalachia.
Very good.
Now explain why it has maintained beyond emigration to the north and for several generations beyond civil rights.
Again --- dialect has nothing to do with "civil rights". It's ******* SPEECH.
I had a gf in California who still talked with the Massachusetts accent she grew up with. And I never once heard her say the words "dude" or "gnarly". These are patterns we all learn in
infancy.
Changing those patterns requires conscious and vigilant work. I had to do it for broadcasting. But broadcasting expects, for whatever reason, a midwesternish "standard American" accent. Among one's own peers, one mirrors. That's another social function.
Now, a black person (or New English person, or Appalachian person, whatever) who wants to go into broadcasting will find it advantageous to standardize, but otherwise there's no reason to.