Rise of Imperialism

With the rise of Imperialism came the need to fuel these Empires with resources. This need prompted the push to explore. Explorers soon colonized many 'new' worlds and started sending resources back to their homelands. This started the need for more workers and cheap labour, and there's no cheaper labour  than slaves. 

Since the White labour and indigenous workers didn't work out (White slaves were, well, White and the indigenous workers often fell sick and had a knowledge the land, good for escape) Europe turned to their partners in trade Africa. 

Europe wanted Africans because:

1) They were visibly different. 

2) They had a predisposition towards farming. 

3) Were unfamiliar with the land. 

It seemed they were 'tailor made' to be exploited.