A relevant idea comes out of the dissonance literature. In a nutshell:

When people feel extremely secure in their beliefs, they care little about what other people say about the issues. If some nutcase was running around on the street talking about how gravity is an illusion, you might smile, and maybe make fun of the person, but you wouldn't feel threatened by the guy's ideology. It is so obviously nonsensical, and the refutation of it is so evident, and your position as a gravity-believer so dominant, that you can be disinterested and above the fray.

When any of these parameters change, people's feelings about contrary ideas change, too. If the guy was talking about something that was not trivially falsifiable, or if the ideology he was proposing could threaten your own interests somehow, were it to be accepted by others, then your feelings on the matter would change. Or (perhaps most relevant to your particular point) if there was a chance that other people might mistake this guy (who is an idiot) as a member of your tribe (which is filled with people of keen and discerning intelligence, such as yourself) then his rantings become a threat, and the job to do is to distance your tribe from his dumb beliefs, and also ridicule his dumb beliefs.

It's informative to consider which exact conspiracy theories draw ridicule from which exact groups. That would probably reveal a great deal about the forces bubbling underneath the surface.