What caused the settlement of more African Americans in the Mississippi Alluvial Plain region then in the Ozark Mountains region?

The reason why more African Americans settled in Mississippi Alluvial Plain is that this region is ideal for agricultural purposes.

The rivers coming from the mountains bring sediments and have built deep layers of soil here making it ideal for landforming. Ozark Mountains region, on the other hand, lack plain land as there are more forests and lakes.

Apart from that, Africans were brought in Mississippi Alluvial plain as slaves and they worked on this land for a long time. That's is why they are more concentrated in this region.