Because not all Africans are of lower intelligence, but the majority are. Human evolution is a very rapid process, the Germanics/Nordics that think they are all high and mighty were illiterate savages just 1500 years ago and were forced into civilizing via brutality. This genetic bottleneck favored individuals who could: delay gratification, process complex patterns, perform abstractions, and plan for the future. "Civilizing" is a real phenomena.

At the moment, we as a society pretend that brains are not biologic and are instead esoteric and spiritual thus we ignore the problem that most Africans do have IQ's far lower than should be acceptable. This is just reality. If not faced head-on with an actual solution, it will have consequences as we have seen.