I’m sure I will get downvoted into oblivion for this, but I’ve gotten a pretty reasonable answer to this question from someone who grew up in the south, and was a descendent of slave owners.

A huge amount of the economy of the southern states was dependent on slave laborers, and suddenly, they were stripped of their investments into those slaves, so they were disproportionately affected economically, and have been behind ever since.

Its likely not the entire reason, but its also hard to see it not being a contributing factor, especially historically.