>>510927516 (OP)Conquest is preferred over slave importation, and extending an empire into Africa before the 1800s was damn near impossible, so anyone attempting to leverage their population or regional connections would be limited to the use of slaves or uneven trade agreements, but since establishing hegemony within Africa is almost equally difficult, there are always neighboring enemies who can be removed to empower the state.
Visually-distinct lower classes are easy to isolate and police. They have no local ethnic ties, no allies, no benefactors, etc.
Finally, and most importantly, Africans are physically and biologically hardy.
In Morocco and parts of Sub-Saharan Africa, this had the effect of making them desirable as a warrior caste to help centralize the state.
In cases, like New World slavery, this not only meant they outlasted the other slaves when in captivity (esp. Native Americans), but were better able to work in poor conditions and resistant to the diseases that now spread around because local mosquitoes got access to African parasites, killing many of the Non-Africans working in those same regions.
It's one of those self-reinforcing things. The more Africans you import, the more you need, and the fewer Non-Africans can survive there, up until either they revolt, or you invent tractors.