Question

the main reason Africans, rather than native americans, became slaves in America was that

Answer 1

Frankly, the Europeans had plenty of time to conquer the Africans. The Native Americans tended to fight back and more importantly, they usually died of the diseases that the Europeans brought while the Africans were used to it owing to the aforementioned years/centuries of slavery.

Answer 2

Yes, diseases were a major issue :)