Yeah, I think the word "phenomenon" isn't quite right here...
...because it's nothing new or inexplicable in Africa.
As in Asia and the ME, human slavery in Africa goes back to ancient times, and probably prehistoric times. And it wasn't (and isn't) about color, it was/is about culture; the powerful enslaving the weak and vulnerable, sometimes to the point of near genocide, and often complete genocide. Dozens of tribes on the Asian, African, and Middle Eastern continents disappeared over the centuries due to the capture, separation, and sales of men, women, and children for the slave trade.