As someone who has been listening to Japanese music since 1999 I have no idea how gangam style made it big in the USA. It's definitely the first Asian song to make it big here. I also listen to Korean music but personally I find it quite difficult to listen to. It's much easier to hear the lyrics in Japanese music so one would think that it would have been some Japanese pop song to first make it big in the USA. There are enough good and catchy Japanese songs out there that one would think one of them would make it to the USA, trust me I know hundreds of catchy Japanese songs.
I used to be huge into Japanese music because of anime. Where they lost me was the explosion of jrock. I was into jpop. So when artists like Ayumi Hamasaki started doing jrock, I kind of left the scene.
Gangnam Style caught on because it's funny with a nice beat. None of the other Korean or Japanese or Chinese stars caught on because they're all trying to be cool. Gangnam Style is this generation's Macarena. If you're foreign, to catch on here, you have to be a novelty.