It actually is a rather interesting question, really, and one that doesn't have the most satisfying answer. It isn't that this is the best cartoon currently airing (I honestly don't believe it is), or even the one with the largest fanbase (I think that, at the end of the day, something like Adventure Time has a much larger group of fans). The Pony fanbase is just a remarkably vocal, obsessive one, and also one that spends a great deal of its time pumping out an absurd amount of art, music, and general whatnot in relation to the show. It's a lightning-in-a-bottle fan movement, and I really can't explain how it got to where it is so fast and so noticeably.