Not a black guy, but I did work in Alabama for a short period in the 90s.
I made some friends, and I asked this very question to one of my friends after I'd been there a few months. This is just one man's opinion mind you, but it was still eye opening for me
I come from a fairly diverse area originally, but I'd never seen so many:
Racist Bigots
and also
Black People
I asked why so many black people would choose to live down there, and he said because down south at least they knew who the bigots were. He said up north folks were a lot sneakier about it.
I would guess that there are more reasons than that, and that those reasons are different for different people. But that was not a response I expected, and I found it interesting.