This is actually the reason. Because there is no such thing as "natural intelligence". Not more than there is "natural strength". There are natural predispositions, yes, but what you get is function of what training effort you put in. Whether you realise, and/or like, putting effort into training your intelligence, is is another thing. So people who are "above average" were in a favorable environment that fostered their development without it feeling forced, or unnatural. And then, when the environment was replaced by the school's, it sadly didn't foster personal development anymore. I would argue we would need to redesign education, now that we have internet. We don't have to design courses around physical limits.