Updated Jul 20, 2019
In no particular order…
Some intelligence is simply ability to memorise. As measured by regurgitating facts on exams. And exams don’t necessarily translate into the “real world”.
Some intelligence is very focused. Such as a person who can write very fluently, but struggles with mathematics that are related to the topic.
Some intelligence is technical, while lacking interpersonal skills/aptitude.
Feeling intimidation and Impostor Syndrome. That can escalate to be deadly (and I mean that literally, and have witnessed it).
Bullying from other people (e.g. employer or supervisor or colleague who sabotages you).
Bullying includes pervasive “tall-poppy-syndrome”, which may come from everyone around the intelligent person.
Money. University funding is fairly generous where I live. But it is still limited. Funding for postgrad study, or scholarships are limited.
Low supply of highly intelligent people, but even lower demand. There are people with PhDs doing the lowest jobs (literally burger-flipping and such) to put food on the table.
Poverty. Some kid from an impoverished background may be a genius, but will be sabotaged by that. Even if they go to university, I have been acquainted with two postgrad students who separately resorted to being homeless (as in illegally camping) while studying. And there are plenty more, in many places.
Drugs (including alcohol). High intelligence may incline people to seek out “evolutionarily-novel” stimulus. I’ve witnessed this one up close and personal.
Intelligence involves questioning. Which involves embracing ambiguity and doubt. Science is all about seeking through questioning. That can lead to a paralysing doubt of one’s self.
Raising the bar. More intimidation and Impostor Syndrome.
And last but not least…
A world full of stupid people. Who feel entitled to all of the things produced/done by intelligent people. While acting with mind-boggling contempt and open hostility.