In my honest opinion, I blame Powerpuff Girls.
As boys, we watched the hell out of Cartoon Cartoons. Latched onto Gennedy Tartkovsky's work with stuff like Dexter's Lab and Powerpuff Girls...I remember there was a quiet, hesitant guilty pleasure among us in middle school, to admit that we were fans of the show...but when we all pretty much discovered, "Oh you like PPG too! Alright! MOJO JOJO, LOLOLOL" and we realize, though it appears to cater exclusively to girls, its totally fine for boys to like it too.
That an entertainment is not segregated to any particular sex. If it's good, it's good. If you like it, then awesome!
The show is adorable. It's really cute, and it's very, VERY, FRIGHTENINGLY EASY to get attached to multiple characters of the show. Every morbid, sexually charged, violently sewn layer of sense of humor built up upon the internet / gaming pop culture, and compounded by shows we grew to love through high school and college...all of it melts away in the bright, happy, colorful universe of ponies. No sane man thinks too long about underlying dangers within the lore of the show...no sexual deviants, no gory, Happy Tree Friends / Retarded Animal Babies crap...it's just pure joy. And that's a kind of a nice change of pace.
It got Bronies because its a flavor of entertainment that piques their particular interest. You go to "Always Sunny in Philidelphia" for a certain flavor of humor and cringe comedy, you go to the Colbert Report for a certain kind of blend of news and hilarity. You can go to Drawn Together for Racism and Homo jokes, or you can go to South Park for a "Tell it like it is" absurd crude-comedy...
I watch Transformers Prime because it's the darkest, most enjoyable animated Transformers series I've ever seen...and on the other side of that coin, I watch MLP:FiM because it's bright, colorful, cute and happy. I like how it looks, I like the emotions it provokes. If I had a daughter, I would absolutely buy the set of DVD's and save them for her when she gets old enough to appreciate them...I'll keep them right next to the 90's "Batman: The Animated Series."