Why are rural black parts of the South extremely rural?

[President Punxsutawney Phil]

They've been bleeding population for decades. This where most of the original Great Migration came from. Including to the cities in the South, they weren't always that heavily black.

There must have been a time when if you told someone that Birmingham, Alabama, would become 2/3rds black sometime in the future, they'd laugh at you in the face and assume you were joking.