The subsaharan region is exceptionally lush and bountiful. There are no real systemic natural problems there. Food literally sprouts from the ground. Animal carcasses from predators are all around. Hunting is very, very easy. Plus, its very hard to move around and dislocate in a jungle, so people can live in places with a lot of isolation between places. There is no evolutionary pressure when the people already have everything they need to settle and survive. Contrast that to Asia, Europe and North America. Even the Amerindians made some sort of civilization with relative structure, because if they didnt do that, theyd die from starvation or freeze to death in the winter. Place that on thousands of years. Their brain is molded that way because of that.