1) There are many dumb people. No amount of opportunity will change that. Ever.
2) Gullibility. A high number of moderately bright, and even some exceptionally bright people have no critical thinking skills. Either nobody ever tried to teach them, or they just don't have the requisite wiring.
3) Homo rationalicus is great at avoiding inconvenient truths.

Add these up and you get a profoundly ignorant population, many of whom will devote their lives to increasing the level of ignorance in our society. The truly amazing thing is that humanity managed to get as far as it has with that much dead weight.