What gave rise to the slave trade?

The main reason why the slave trade developed was because of the colonization of America. Initially, the European colonists tried to use the Native population as workers. However, they were not immune to European diseases, which caused them to die in large numbers.

European settlers, therefore, turned to Africa  in order to find cheap labour. Africans were well-used to European diseases, as well as to the business of trading with slaves. The trade grew as the demand for labour in America continued to rise.