Why do spanish colonies begin to allow more heavily on the atlantic slave trade by the mid-1500s

Spanish colonies begin to allow more heavily on the Atlantic slave trade by the mid 1500s in a few reasons. Before Spain aggressive act, Latin American civilization prosper. These four main ones was Inca, Aztec, Maya, and Olmec. After Spain found them, everything changes. They began to colonize them. Even though they are Latin’s most richest civilizations, they couldn’t defeat them. Why? Because they lacked technology compared to Spain’s military weapons and such. Which the start of economienda began. A same thing for mercantilism. And social class develops. However as they colonize the whole South America, they used tons of Hispanics to work for them. But as the disease transmitted to them, more and more people died and the population decreases. As the disease had gotten to them, Spain was in desperate situation. The reason is because without those people, their colonization would be crumpled into pieces and they need to someone to continue their colonization. So they turned to the Africans. Where they need them to prosper their colonization.