What factors that contributed to the predominance of African slaves in the Atlantic slave trade?

The new settlers of the Western Hemisphere initially used the indigenous people to be their farm slaves. most of the new slaves got too sick and died with new European diseases. The settlers then needed a new source of labor. They traded with African tribal chiefs and kings for their own slaves. More Africans were sent to modern-day Argentina and Brazil because of the diseases and cruelty of the new territories. At least three times as many Africans were sent to South America than its northern counterpart.