Look, I've been living in Japan for a while now and I've talked to alot of Japanese gamers about this. Never once have I heard them say "it's not popular because it's American." Contrary to popular belief, they love alot of American stuff over here. My wife is Japanese and we have all 3 consoles here at home and I asked her which she liked the best. Her reply was the Playstation because of Ratchet and Clank and Mainichi Ishoo (weird news program with cats). From what I've seen, the Japanese are attracted to cute, lovable characters (like Ratchet, Mario, the Final Fantasy characters, etc) and the 360 just doesn't really have that. Sure, there are a few games that Japanese people are interested in (my bro-in-law loves Fight Night), but when an overwhelming majority of the games are FPS and games fueled by testosterone, they start looking at other offerings.
Microsoft got off to a bad start in Japan and it's too late to recover now that the PS3 is out. If MS wants to make another system, they better start encouraging developers to make a wider variety of games and they better find a cute, lovable mascot or it just won't sell in Japan.