Why did the colonists in need of cheap labor turn to slaves from west africa?

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.

When the Colonists arrived in Africa, the legal system in Africa was fluid in nature which they moulded according to their convenience. The increase in the profit of plantations required more laborers which the Colonizers got steadily from African Countries. They required cheap labors in every sector of the Economic activity, for example, to run industries, for export and import and for plantations. This requirement reduced the position of the African people from being cheap laborers to slaves.