There is an entire subculture called 'bronies' of adult fans of the show. My best guess is that the demographics of LessWrong readers come closer than average to the demographics of bronies, and so there is a higher-than-average chance of a LessWronger being willing to be a brony.

As for why the show is popular among adults; the best theory I've heard is that unlike many modern cartoons, it's unashamedly non-cynical, and is actually reasonably-decently written and produced, which makes it near-unique at the moment.