I don't know. It's like any other show, I watch it and I either click with it or I don't.
I stumbled across a PMV on youtube (You're Gonna Go Far Kid) one day, then I researched about the show and bronies. Thought nothing of it, then about a month later came across another PMV, decided to watch the first episode, then well watched the whole series and here I am!
The way it looks, the way the characters are, how they sound, the humor, the references, it all contributes.
TL;DR: PONIES OMG