Because the Columbian Exchange brought disease to the Americas and killed most of the work force enslaved by the Europeans. They started using Africans. since there had been much contact between Europe and Africa disease wasn't really an issue. many tribes in Africa sold their countrymen into slavery for weapons and other status symbols. One of the good things to come out of it was the introduction of new cash crops to Africa. Because of the Columbian Exchange, Africa and the rest of the world has potatoes and corn as well as many other crops.
