We'll admit we've often asked ourselves this same question ... but now some actual scientists have gone ahead and put the scientific stamp of authority on the notion that if a person is kind of dumb, they are not capable of judging whether another person's idea is dumb, and therefore they are bound to make a stupid choice in an election.
Also, "with incompetence comes the illusion of superiority."
Cornell psychologist David Dunning and colleague Justin Kruger are responsible for the study, reported on by LiveScience (and it's making SFGate commenters go batshit, of course), and a sociologist in Germany has already used their findings to build a computer model of a democratic election, showing that people will always end up electing someone with "slightly above-average leadership skills," as opposed to people with really great leadership skills, because most people aren't good leaders and aren't capable of recognizing them either.
Anyway, we just have one word for you to support their thesis: Santorum.
[Live Science]
[SF Gate]