Why were africans traded as slaves?

The short and simple answer is that they were not seen as humans, but as savages. The Africans that the pioneers found in their travels were usually simple hunters. They were capable of building only simple structures, hunting and gathering, and growing crops. Their social structures, if any, were unsophisticated compared to Europe and Asia. The pioneers saw this as a great opportunity for very cheap and abundant labor. They proceeded to ship the Africans back to America and made them do all of their hard labor.