The answer is incredibly straightforward - the population (heritable) intelligence distribution is vastly lower for most African populations than for populations in europe, european-colonized south american regions, most Asian regions, etc. Anyone who can dispassionately perform bayesian reasoning here will settle on this as the most likely hypothesis, by a wide margin. It's unfortunate because there's not a straightforward fix like "give them better schools", so for this and other (mostly religious) reasons, people don't like to believe it.