the best way to explain it is that it's a boys adventure cartoon that looks like a girls friendship cartoon. girly cartoons don't have shit like fighting ancient evils, or searching for hidden treasures, or traveling across the world on to rescue a friend captured by bad guys.
although I guess part of the allure is also in how inspired the fanbase is. there are entire backstories and semi-canonical characterizations for ponies that were just meant to be one-off background characters. People have written entire original pieces of music inspired by the show, there's a strong remix community with work all over youtube, Deviantart is probably half ponies at this point.
It's not all one-sided. in the second and third seasons, some of the fan-based characterizations actually became canon, with the official recognition of derpy hooves and others. It's a mutual give and take between the creators and show runners, and a fanbase who aren't a bunch of little girls about admitting they like a show about adventure and friendship and self-discovery.
ALSO this: Doctor Whooves / Time Turner | Know Your Meme
and this
