I guess one of the reasons could be Kinect's space requirements. If I remember correctly, many japanese were concerned that they might simply not have enough space for Kinect because of rather small japanese rooms and flats. And that was before Kinect was even released - when Kinect was finally released, the space requirements indeed quickly turned out to be the most discussed problem, even in countries where people didn't expect this to be much of a problem.
And apart from that, Kinect doesn't actually do THAT much worse in Japan than it does in EMEAA for example. In Japan, the Kinect/Xbox ratio is about 11.5%, in EMEAA it is about 14.5%. If it's true that Japan has a higher percentage of core gamers, then that alone might be the reason for the 3% difference.







