Slavery has gone on for years , in some arab states they still have African slaves.
As for European white slave traders is was as simple as being more advanced than the black Africans. What I mean by that is - better ships / weapons / etc.
They wrongly saw the tribes people as animals to take advantage of, this is a part of world history most people are not proud of.
The fact is it happened and people got rich off it, on a small scale it still goes on today.