Otherwise it's difficult to question everything you thought you knew about anything. Normally it's not a pleasant experience. That is the reason that so-called enlightenment happens only to those who had big inner struggles after many years of reflection.
In a Freudian way their unconscious side protects them from threats attacking their mental health. Knowing what's right and wrong is necessary to do anything with reason. If you accept the fact that there is no right and wrong, you can definitely lose some motivation which is detrimental to the incentive system that our consciousness gets trained with.
It's therefore rational for most people to reject those ideas (although they don't necessarily can reflect on this level). I wouldn't call it "intellectually handicapped", I would say their mental processes protect them very well from thoughts that could lead to a loss of their values and belief system. Otherwise it sounds so negative although it's a rational thing to do for the consciousness. Normally your incentive system is not "I want to know the whole damn truth", it's "I want a happy and fulfilled life, want to feel good about myself and achieve something". Cognitive biases help us to achieve those goals. It just seems irrational from external point of views because of missing profundity in the analysis of the psyche of the other person. They're perfectly logical w.r.t. the incentive system.