What was one main reason why slave trade greatly expanded beginning around the 1500s?

Slave trade in the 1500’s expanded coincidentally at the time where Europeans where discovering or conquering new lands in the Americas. Well, the Europeans had already decimated the native populations in the Americas and so have so much land yet so few people to farm or develop it. The solution is to import slaves from Africa, so they tapped the already-established slave trade there. The trade proved very lucrative to slave traders, who set out more expeditions to capture slaves in Africa to sustain the increasing demand from the Europeans.

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