I have a theory on Brony culture, one I will admit is likely quite defeat-able, but still potentially valid:
Even if there has never been a singular, specific admission by any significantly large number of people; the collective consciousness of these entertainment viewers has finally reached the critical point where it can no longer stand the drought of product catering to their specific tastes. This is akin to 'cultural starvation' for these individuals.
When you are literally starved, even a dry cracker tastes like the finest food ever consumed. Your body will devour it; in fact the impulse to swallow it whole may not be within the bounds of voluntary control to resist and eat it like a 'civilized' individual. You might've wanted (your specific tastes) something more fitting to eat than a cracker (something that grazes the edge of your peripheral interests despite not being the target audience), but at that point you cannot reject the survival impulse to eat when food is placed directly before you.
My Little Pony is this cracker.
The 'brony phenominon' is the involuntary disembowelment of the cracker; as observed from an un-starved individual.
I myself am not a 'brony'. (Ironically, despite being a writer, I have a deep-rooted hatred for fan communities* and what they represent these days.) But I think I can empathize with how 'wholeheartedly' they dive into the material when viewed from that perspective.
*Meaning more the mutation that 'fan' has come to represent rather than the traditional definition.