Where I'm from, it's because of coal mines. They saw their parents and grandparents do reasonably well (financially; health is a-whole-nother story) in the mines without a degree or with only a basic degree and they accepted that school wasn't necessary. Now you have people telling them it is, and doing better because they had more education. It gets to the point where they see that as not trying to screw them over but as elitism, so they blame those people that the options of the past aren't there anymore, and look to try to make themselves feel better.
Though lots of America is fundamentally based in anti-intellectualism. Anti-Intellectualism in American Life was an eye-opening read and helped explain a lot of what I saw around me in the rural Bible Belt.
