The graph is, in a large way, an inversion of population density.
I’ve outside of this heard the South has in someways never really bounced back from the end of slavery. Could be a kind of cultural scar or lack of “infrastructure”. Like how Lincoln’s reconstruction plans never happened and his successor went a quite different direction.
At least anecdotally it feels that way.
I moved from one of the states that was totally clear in this map, to eastern Texas when I was younger and the social scars from the civil war still exist and by the way some people talk even to this day, you'ld think the war had ended last year. It was complete insanity to me.