it's just that just about nobody has even tried to make a tv show for little girls that has even an ounce of appeal to any other demographic before.
The Powerpuff Girls. That show could be enjoyed by both little girls and other demographics - and was enjoyed by multiple demographics. Totally Spies may have been another show like that (but I've never seen a full episode of that, so I'm not sure about this). Of course neither of these shows were as big as MLP is.
What's different is the reasoning adult males are giving for liking the show - they watched PPG because it was funny, but a lot of them are saying the appeal of MLP is that "it's so cute and happy." Never seen that before. And no, I'm not being critical - I'm glad adult males are able to enjoy cute and happy things.
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Full rap metal jacket ~ Method Man