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.