The system is rigged. Plenty of Africans have the intelligence to see through the game, but there are always power-hungry individuals who'll act in their own interests over those of their people. The Euro colonialists did a great job of setting up Africa to fail. Some of this is explained by Chinua Achebe in "Things Fall Apart". The English found the more dubious individuals in Ibo society, and put them into the gatekeeper positions. The traditional power structure was ignored. So... you get people with influence who are beholden to the colonizer, who were already somewhat alienated from their own people, and who therefore are more likely to act in their own an in the colonizer's interests than those of their own country.
If the intelligent. and more to the point, ethical, people make too much progress, they are "dealt with"... Lumumba, Obote, etc.