A lot of these populations of undereducated are in areas that culturally oppose outside influences. See: cults, extremely rural Appalachia, pockets of immigrant groups opposing assimilation violently in European urban areas. Education = indoctrination to them and a loss of their cultural values. How do you teach people that may need it most when they don't want it?

This is not the case by and large but it puts a ceiling on the efforts to educate away problems with the Idiocracy trends of the world.