Why did the english prefer to use slaves from africa rather than native american people?

One big reason was that there were not as many native Americans: many were killed by the diseases broight by the Europeans.

Another reason was that the Native Americans could escape more easily, since they knew the land and they could hide among other Native Americans, while the Africans could not escape easily - and they could be found  easier, since people could assume that all black people were slaves.

He sailed off course on his way back to greenland and ended up in north america