I know what you're trying to say, but you put it together so wrong that it sounds like you're saying that Japanese music isn't real music. That's how little logic the post makes, but from what I could decipher, you're implying that the majority of the game revolves around Asian songs, whether it be pop, rock or any other genre.
The only logical answer would be that you're fundamentally wrong, as you failed to comprehend that the game is what it is due to the community. The community made the game that you know today, and from my point of view, that is one of the good aspects of it. Of course, it is completely subjective, just like your own opinion that you expressed there.
The other, less logical answer would be that you haven't looked into it enough, to discover that there are hundreds of songs that have nothing to do with neither Japan nor Asia in general, making your whole question and point invalid.