African-Americans with slave ancestry almost all come from West Africa I would think, as that was the shortest route for the European slave ships between Africa and the New World.
There are a number of African-Americans with Malgasy descent. It's true that the percentage would be small compared to the percentage of African that came from west Africa.
Bottlenecks, small population inbreeding, genetic drift, founder effect, etc
Basically what happened was, an alt-group of African humans decided earth was not safe anymore and its collapse was imminent -- they need to get to Mars. But since they don't have a way to get a spaceship in Africa, they will need to leave Africa and do whatever is necessary to build one, including throwing the people who helped them under the mammoth if need be.
At this point they have a pretty diverse gene pool and represent the humans well. They walk for a bit but came upon some steam vents and decided take a break and steam some food and enjoy the ocean view in the distance. The leader is not happy they stopped for food or rest. Later they are shooketh awake just before fire rains down on them. 3/4th of them make it safely to the ocean, and what luck the water is receding it'll be easier to cross, for the first 2/3rds at least. The tsunami gods demanded a toll and claimed the back 1/3. Not happy with the direction of leadership, half the group decided they could establish a better foundation away from these clowns and forked themselves.
Meanwhile, back in Africa and finally having got rid of that crazy mars dude and his cult followers, they decide to invite all the neighbors to an epoch swinging dance party to celebrate -- complete with the best plant-based party favours they could dig up/gather. Thus, ensuring the larger African population and their descendant would end up with an abundant supply of pokemon variants to swap between'em.
Though, you may want to take a look at the linked paper above for a more precise explanation. I may have paraphrased a bit.
...which needs more to explain it, such as (if true): "the diversity originating in Africa did not migrate out of Africa, and the gene lines that did leave Africa did not develop as much diversity as had existed in the ancestral gene pool, despite facing far more diversity in climate than exists in Africa".
And then I would again ask of the latter, "what? Why not?"
Ultimately: Why did Africa stimulate so much gene pool diversity? It seems more homogenous in environments than the rest of the world that Homo Sapiens emigrated to.
And there would have been cross-breeding between African subpopulations. Environmental barriers in sub-Saharan Africa are minimal in terms of gene movement, aren't they?
Perhaps there simply hasn't been time. Large scale migration out of Africa is a recent phenomenon relative to the existence of anatomically modern humans.
