Why? Why would a show aimed at little girls attract such a large audience of non-little-girls? Most bronies would give a simple answer: “This show is awesome”. But it goes deeper than that.
Clearly it’s a good show, with good characters, good writing, and good animation. But that hardly explains the scale of the brony phenomenon.
The big factor is the brony community itself. The community sustains and expands itself, and the show has become a proxy for a variety of values and relationships that are much more important and fundamental than the actual show. Allow me to explain:
Though we don’t often realize it, young men (approx. age 13-29) are constrained by a variety of social norms. (The same applies to any other demographic, of course.) We tend to enjoy badass things like superheroes and martial arts. (Well we’re mostly internet nerds so we don’t do much martial arts, but we watch martial arts movies etc.) We spend a lot of time playing Halo or Modern Warfare, killing aliens and terrorists. This is all fine; we actually do enjoy this stuff. But for most of our lives, we have been socially prohibited from enjoying anything cute, pure or innocent. (Even our comedies are crude.) This prohibition is so strong that we don’t even realize it’s there; we don’t even think about various styles of entertainment which don’t fit within our social norms. (And those who deviate are often derrided as “fags” etc.) This has created a kind of emotional malnutrition; we have not allowed ourselves to consume as much cuteness, pureness or innocence in our media as we would actually like to consume.
MLP:FiM contains all of these elements in spades, with the strong bonus that it’s a well-made show overall. With the advent of the brony community, young men have collectively granted each other permission to rewrite our own social norms and relieve our malnutrition. The existence of the community encourages new bronies to “come out of the closet” as it were, not to admit that we are gay (which we typically aren’t), but to admit that we enjoy these elements which are typically regarded as feminine and/or socially prohibited to us. Thus the community enables our enjoyment of the show, and the exhilaration we experience at successfully changing our own social norms is one of the primary feelings that we share with each other. (Though most people don’t think this through in such detail, and reduce the entire phenomenon to “This show rocks!”)