The main reason why the Spanish began importing slaves from Africa to work in the Americas is that they needed people to work in the colonies, and the Native Americans could not fulfill the demand.
Natives were dying very quickly due to exposure to European diseases. As they continued to die, there were not enough people to work the land. Africans were resistant to European diseases, and they were familiar with European farming techniques. This motivated the Spanish to begin importing them as slaves to work in the Americas.