Everyone knows MLP's greatest attraction is its shock value, like all memes.
It's a hipster thing. "hurrr this show's targeted toward little girls and I'm a guy watching it isn't that ironic?"
The fact that its mass popularity has shrunk over time is testament to the fact that once people stopped giving a shit about grown men liking a show about pony-girls, it lost some of its appeal. Whether the show itself is actually good or not doesn't really factor into the equation a whole lot. There are a lot of really good shows out there, but you don't see most of them having the viral/meme level popularity that you see with Bronies.