Why did the Spanish enslave Africans?

The main reason was that Native Americans died in numbers that were too high (especially in the Caribbean) to become a viable labor force. The Spaniards were aware of the method of enslavement from Islamic Spanish States which had Black African slaves and knew the quality of those slaves. Therefore, it was logical to enslave Africans to build the colonial enterprise.

The idea that slavery is fundamentally wrong is entirely modern.