It's because there has always been an anti-intellectual streak in American society, and this is seen elsewhere in Anglo societies such as Australia, because being intelligent and intellectual is seen as being elitist and snobbish, or even anti-democratic (since the idea that a person knows more and therefore their opinion has more weight means there is inequality). This reaction then is easily changed to a desire to take these snobs down a peg, in other words "tall poppy syndrome". The Republican party is particularly all in on this as this leaves the masses ignorant, while the masses distrust education because it tends to turn people "liberal" (as in they get a broader global view and often don't agree with the small town parochial view of their families).
