Western kingdoms in Africa were successful and became overpopulated quickly. Overpopulation created a flow of migration to other areas of Africa. Since these people migrated to the east and south, considering new resources and developments that those people brought with them, these areas where people migrated to ended up becoming way more developed very quickly.
Considering that, it wasn't because of people from other continents, the search of new fertile ground or the end of the resources in the Western Kingdoms, but the overpopulation. The answer that correct represents this is the following:
As the populations of western kingdoms grew, overpopulation caused citizens to migrate east and south to build new empires.
