Why did some colonists enslave native Americans and africans?

Because it was cheap and effective labor, they tried with the native americans but the native americans could not handle the high work load and were susceptible to various diseases, africans on the other hand were more capable of handling the high work load and were less likely to die from certain diseases than native americans. And to those that try to say Africans sold Africans into slavery...you have to remember that africans sold their enemies and not their own kin and moreover Africans did not create the large scale tranatlantic slave trade that led to all those tragedies.