The great migration is part of it. Staying together segregated is the other part.

My mother's family is from Boston, My Father's family is from Oklahoma, I don't sound like Chicagoans whose family is from Mississippi. I grew up in Chatham. I still sound like a Chicagoan.

If I came home from school speaking like my classmates it was corrected immediately. I could have picked up some of that accent or way of speaking from my peers. My grandmother made sure I did not.