What was one main reason why slave trade greatly expanded beginning around 1500s?

Most European countries were in expansion of conquest, and Africa was a found ground for it. Many Africans were kidnapped from Western Africa and brought to Portugal. But a reason can be for the economic prosperity the slave trade brought to those countries.

The European colonizers had enslaved the native people and were using their labour, but the Colombian exchange had brought many diseases that the natives had no protection against and they died out pretty quickly, so the Europeans began buying slaves from Africa.