In the 1800s and probably even before that people were enslaved in order for certain people to get what they wanted. For instance Europeans came over to the United States and acquired land. One way that they got this land was to force Native Americans off their own land. Another way that Europeans who now became citizens of the United States were able to get richer was by purchasing slaves.
It originate it by leaders and property owners going to Africa and giving the African tribal leaders money or goods that the leaders deemed valuable. In exchange the African leaders gave them the people of their villages to be slaves. I think the main reason that these African tribal leaders gave people was for the money and to get richer.