Why did europeans prefer to enslave africans rather than indians?

Answer 1

Europeans preferred to enslave Africans rather than the native Americans because they considered Africans a hardier people. Native Americans were susceptible to many European diseases such as smallpox and measles, while the Africans didn't die as much from these illnesses.

Answer 2

It depends on the timeframe in question.

Prior to the 1530s
In the first few decades of European presence, large numbers of Native Americans were enslaved and forced to work on European projects and there was no conception that Africans should be imported to take on labor functions. Columbus himself was infamous for the brutal way that he treated the Taíno people, the natives of Hispaniola island, forcing them into slave labor in the recently excavated mines. The work was so callous and so brutal that significant percentages of the Hispaniola Taíno population died from overwork and fellow Spaniards reported him to Queen Isabella, who was in turn so enraged at the conduct that she demanded that he return to Spain. (Remember that this is the same "humanitarian" Queen Isabella who is responsible for the Spanish Inquisition.)

The Native Americans died in record numbers (especially in the Caribbean) within decades of the Spanish arrival, primarily from foreign diseases, but also from overwork. The Spaniards needed a labor force to fill the lower rungs of the social ladder in order for the colonial expedition to be profitable. Since the enslavement of Africans was quite accepted in Spain due to the former Islamic rulers having African slaves and participating in the African Slave Trade, it was only natural for the Spaniards to expand this trade and begin to use Africans to replace the dying Natives.

In places like the Andes, where the Native American population did not die off rapidly, there was never a shift away from using Native American slave labor. This is why Peru and Bolivia have a very small percentage of African descendants today. However, after the 1530s, these laborers began to receive compensation (although, nowhere near commensurate with their work and nowhere near sufficient to survive).

After the 1530s
There are two major reasons that there was a shift to exclusively African slavery and the massive expansion of the Trans-Atlantic Slave Trade.

Practical Reasons
The Africans had much better resistance to both European and tropical diseases since many of these diseases had become prevalent in Africa and, like Europeans, the populations that could not handle these plagues had already been wiped out. The Native Americans had no prior exposure and the immediate genetic winnowing easily removed over fifty percent of the endemic population. Additionally, Africans were far more used to heavy agricultural labor (the plantation system for the cultivation of tobacco, cotton, and sugarcane) because the crops grown in Africa were similarly laborious, while Native Americans (especially in the Caribbean) were not able to survive the more strenuous labor requirements.

(As noted above, in places where plantation culture was not dominant and where Native Americans were not eviscerated by disease and overwork, Africans were never imported en masse.)

Religious and Legal Reasons
There was generally more sympathy towards Native Americans and their tribal societies than there was towards Africans. This manifested in an edict from Charles V of Spain in 1530 prohibiting the enslavement of Native Americans and the Papal Bull of Sublimis Deus prohibiting the same in 1537.

The Papal Bull divided the world racially and argued that the Native Americans were simply innocent people, as if they had just walked out of the Garden of Eden, and deserved the chance to "willingly" convert to Christianity and achieve salvation. Conversely, it argued that the black skin of the African was indicative of the dark nature of his soul and indicative of the fact that the African could never truly be redeemed by Christianity. As a result, enslaving the African and forcing him to live among good Christian Europeans would be a net positive for the Africans. (Of course, Catholicism has since changed its perspective and currently believes in racial equality.)