Immunity. Africans were immune to diseases that the native Americans. So instead of being able to labor, the natives were getting sick and dying. The Africans however were considered incapable of taking care of themselves, strong beings that could sustain hard work and inferior. Also if you take the Africans away from their land they were More likely to work out of fear of being in a foreign place forcibly rather than a native who is already on their own land.
