There were a couple of reasons why Spanish settlers turned to African slaves to provide labor for their american colonies:
1) The Indian slaves that they had there were already decimated by the diseases brought by the Spanish conquistadors, which left very few of them. As production rates increase, the Spanariads found it harder to find more workers.
2) The African slaves were MUCH more cheaper then if they hired workers from Spain, for all they had too do was go there, take them away by the gunpoint, and then bring them over too the Americas too be sold. (not saying its a good thing, but facts are facts)
etc etc..
hope this helps
