I personally think it's not popular because it looks "childish" to a lot of people, and rated M games are all the rage. Animal Crossing has more of a cult following, in that although it isn't making record breaking sales, you can bring the game up in conversation and people will generally know what you're talking about.
But it's never going to blow up to Mario sized proportions because the western world tends to regard anything that's rated E to be stupid, which is unfortunate. Mindcraft's popularity has given me a little hope though, and with Smosh reviewing the game, I could see New Leaf making a name for itself.
