Most Europeans didn't rule their African possessions directly, but rather through the proxies of local rulers. And once Europeans left, those local rulers, the upper classes, were seen as illegitimate collaborators. This meant that a new group of rulers had to rise up to take their place, often with very little experience in governance. I mean, Zimbabwe's long-serving dictator Robert Mugabe was a high school teacher. Let that be a lesson to you. Your teachers may have dictatorial ambitions. But most strongmen have emerged, of course, from the military.