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).
So now we have a society in which a significant segment believe higher education is not only unhelpful but actively harmful in brainwashing the children to be liberals. It is of course a recipe for long term disaster and ceding of technological leadership to others.
This attitude is not only seen in poor white people but also in Latinos and blacks in the United States. Getting educated is seen as being a betrayal of one's roots, turning "white" or collaborating with the white power structure, and is discouraged or the person is ostracized by their peers and family. It is all part of the so-called "culture of poverty", which basically refers to the culture or beliefs that are self-sabotaging and lead to perpetuation of inter-generational poverty. The left in the 70's attacked this idea as being racist and victim blaming, but I think it has merit and goes beyond race and affects any group such as the stereotypical Southern white "rednecks".
