Let's try putting it this way - how is an African empire going to expand across 4/5ths of the continent before 1900?
Alternatively, how is one colonial power going to invest into conquering & controlling an entire continent twice the size of their own while also preventing any competitors from establishing a foothold? I mean, the Sun Never Sat on the British Empire and all that, but Paris' African empire was the same size and extent as that of London's. Hell the Germans got into the colonial game long after any other colonial power and it's African territories were still roughly half the size of Britain's.
It's conceivable that a civilization similar to the mongols could have arisen in africa. The songhai, mali or ghana empire could have gone another way and focused heavily on territorial aggrandizement. It's less ASB to have this be done by an internal african power than by a european one and to be fair having a european one do so IS asb. Its impossible to forsee a scenario where one european power develops at a faster rate than the others in time to conquer the whole continent and post scramble for africa its also impossible because european countries would never go after each other's colonies for the same reason they propped up the ottoman empire: to prevent a general european war.