Final answer:
The settlement of more African Americans in the Mississippi Al`lu`vial Plain region than in the Ozark Mountains region was primarily caused by economic opportunities and social conditions.
Step-by-step explanation:
After the Civil War, many African Americans sought refuge from racial violence and economic hardship in the South by migrating to other states and territories. The Mississippi Al`lu`v`ial Plain offered fertile land for agriculture, particularly cotton production, which attracted African American settlers.
Additionally, the fluid social hierarchy and complex relationships between the Five Tribes and their Black population in the region provided opportunities for African Americans to maintain their cultural heritage. In contrast, the Ozark Mountains region had less agricultural and economic potential, which may have contributed to fewer African American settlers.