The 360 does NOT have the kind of popularity in Japan that it has in the States and some other countries. Not even close.
I mostly think some games get 360 releases over there at all just because they will have more success/greater profits on the international market. The Wii was the most popular console until just recently, as Cyber mentioned above. Nintendo and Sony pretty much own the gaming market there, though for awhile Sony wasn't as high as Nintendo was by far.
I remember there were atleast a couple OP PS2 games. Is that right? Otherwise mostly everything else I can remember was on Nintendo consoles, it has been more recently that PS3 popularity is exploding and now we're seeing OP games there too. It's just a case of different market culture, the 360 doesn't have much clout in Japan as it does back in the West.