Probably the main reason africans rather than native americans became slaves in america was that

There were a couple of reasons:
1) the Africans looked a lot more different from Europeans than the native Americans (so they could not "blend in" so easily
2) The natives would have an incentive and a place to run away- and a support of a whole tribe. But once you shipped the Africans away from their country, there had no place familiar to them to hide in.
3) Europeans and Africans already shared immunological defense to the same diseases 

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.