Answer;
-West Africans were already to the Europeans diseases
Explanation;
They originally enslave American Indians, but three things put a stop to it. First, the Indians were all dying of epidemic diseases, which Africans had already been exposed to. Second, the Indians, being native, had a better knowledge of the land and its peoples, which made escape/revolt attempts more likely to be successful. Third, in response to debates about Spain and Portugal's treatment of indigenous people, the pope issued a bull in 1531 that banned the enslavement of American Indians.
As to why Africa, slavery had already existed in Africa for centuries. The Spanish and Portuguese had been exposed to it through their contact with the Moors of North Africa. So when they needed more labor in their American colonies, they simply bought slaves from one of the various kingdoms of West Africa and shipped them overseas.
