The reason why much more African Americans have settled in the Mississippi alluvial plain region than in the Ozark Mountains region is the agriculture.
The alluvial plain region of Mississippi has very rich and fertile soil. Because of that, this region has been turned into farmlands. Since the African Americans were initially brought as slaves to work on the farmlands, it is no wonder why they have been much more concentrated around this fertile plain.
The Ozark Mountains region, on the other hand, hasn't been exploited agriculturally because there hasn't been proper conditions for such a thing. Because of that, the African Americans were not settled in this region, so their concentration is much lower.