Board Thread:Questions and Answers/@comment-26105630-20150213141049/@comment-1268838-20150401012159

I think it's possible that Karakura Town has been the juureichi for such a substantial period of time, that the vast majority of individuals with sufficient spiritual power to become Shinigami are Japanese... That's the closest thing to a plausible in-universe answer I can come up with. I mean, there are still a few individuals that are blatantly foreign. Though frankly, it's more likely that it's just an arbitrary choice since Kubo is (obviously) Japanese himself.