Europeans turned to africa as source of enslaved persons because

Groups of American Indians were exploited and decimated

Explanation:

When Europeans came to America they were engaged in many wars against the natives here, this meant they killed and exploited many of them, when they needed workers to exploit the natural resources there were non left, this meant that they had to turn to Africa to trade slaves and bring them to America to do the work for them.

B-groups of American Indians were exploited and decimated.