Why did the use of african slaves increase in the americans during the 1500s

Mexican graveyards are the only evidence which connoted the existence of slavery in America in 16 century. The records which had the count of the slaves transported during the 16th century were less. But from seventeenth century, the records are available.

There was a massive depletion of population in Africa due to the export of slaves from Africa. In order to protect themselves from ruthless Europeans people started to keep them armed with lethal weapons  

There were huge internal conflicts in Africa which led to increase of warlords who failed to establish a free state. This caused enslavement of Africans.