I was in the South in 1950’s and I can attest to the fact that even them the parts of the South that Isaw was still a backwater
It was a backwater, not because of what the US did 100 years ago, but because of what the people who lived there didn’t do.
Perhaps you think that illegal lynching of scapegoats as an accepted practice would lead to rapid growth? It doesn’t because investors and enterpreneurs knew that the same forces could be unleashed against them. The KKK was deployed against blacks, but also against people from outside the south who sought to develop it (aka carpetbaggers) and people from the south who sought to develop it (aka scalawags).
So development was not permitted. That led to the slow growth, or even refusal to maintain what businesses were there.