I think I finally figured out just what it is that caused Friendship is Magic to develop such a following.
The kinds of people that are fans of the show were also fans of 80s and 90s cartoons when growing up. Quite frankly, most of what is released today absolutely pales in comparison to what was released back then. In fact, "good cartoon" has become an oxymoron lately.
And there's another thing. Just because we're adults doesn't mean we magically stopped being fans of cartoons. Cartoons helped raise us. They're a part of our identity, our culture.
Friendship is Magic is rare in that it's a cartoon that
doesn't suck nowadays. It's nothing too innovative (other than being the first girl's cartoon since She-Ra to not suck) but in a way, that's why it appeals to us.
It is what we have been missing. A decent cartoon for us to enjoy. To remind us of the Age of Imagination that has faded away.
Quite honestly, except for nostalgia channels, Friendship is Magic is superior to most cartoons on television today. It actually manages to be more clever, witty, and funny than shows that are actually directed at adults (when cartoons aimed at adults are dumber and less mature than a show aimed at little kids, there's something fucked up in the world)
...It's kinda odd, but MLPFIM is actually more mature and intelligent than a lot of cartoons today (and, to an extent, more intelligent than a lot of other shows, too.) Which is part of why it's so good - it's not condescending towards its target audience.
It is an oasis in a desert of bad television. And animals all like to gather together at the watering hole.
That being said, I still miss the Age of Imagination, but Friendship is Magic helps a bit with that.
Now... is it a show aimed at little kids? Yes. We bronies are a periphery demographic. Should we feel ashamed for watching it?
Nope. C. S. Lewis said it best.
Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.
"On Three Ways of Writing for Children" (1952)