It comes back to a tendency that I feel more and more is the most pernicious influence of all in the public sphere - the inability of people to believe that those on the other side of a debate are reasonable, rational, intelligent people who are doing their best to solve difficult problems. We (and I include myself here) have a deep-rooted impulse to characterise the other side as either charlatans, crooks or imbeciles.
Taking someone seriously who has reached different conclusions to oneself or has a different worldview triggers such cognitive dissonance that we'd prefer to ascribe to them either nefarious motives or else some kind of mental deficiency.