Think about it from an evolutionary aspect. Sub Saharan blacks never developed above stone age hunter-gather because they can pick their food from trees, had widespread access to game, and never faced any kind of seasonal pressure that forced them to plan for extended periods of no food, like whites did in Europe, so there was no evolutionary pressure to get smarter.

To put it in software development terms, Blacks are the alpha version of humans, closest to the proto-humans modern man first evolved from.