I think people like ponnies because 4chan told them to like ponnies and they can't decide whether or not 4chan was being ironic. They don't want to be left out because 4chan is COOL! It's important to prove to the world how much you really really like ponnies if you want to be COOL too.

That's the why, but I don't really have a problem with people liking ponnies though. It's certainly one of the less annoying fads to come from the internet. The animation looks kind of neat too I guess, although I've never been interested enough to watch an episode.