My take on the matter is this, I may get flamed slightly for my opinion, but yeah.
Basically, guys have their girly side, too. For me, this is expressed in a lot of things I do. A vast majority of the music I listen to is girly pop music, ie, eurobeat, happy hardcore, http://www.youtube.com/watch?v=GUwIv3fuZ0g Like that. I'm guessing my reasoning for liking it is my emotional maturity is probably quite low, and I'm quite alexithymic, I usually don't know how I feel at most times. But songs like that for some reason, I feel I can "connect" much better to.
So with MLP, it's not really a bad show by any means, it's done by the same lady who made the Powerpuff Girls, it's funny at times. But I myself really don't see the big deal. If I watched TV and it was on, I'd not turn it off, but it's not something I'd specifically seek out and watch.
But, what I think it is, it gives guys who wouldn't otherwise have an outlet for their "girly" side an appropriate outlet. How it got appropriate is, probably a few guys who legitimately liked it started posting it on 4chan, Reddit, whatever sites those hip young kids (lololol) post on nowadays, and then out of curiosity, other people watched it, and followed them, got on the bandwagon if you would. So now, many guys who have inclination to watch such things as MLP, but wouldn't because of peer pressure to not do it, now have peers that watch it, so there's solidarity in their group.
That's what I think.