I don't know for sure- I think it has to do with a few things. Japan was fairly early to jump onto snowboarding, only a few years after the sport launched, since Japan is a big winter sports country.
Back in the early years of snowboarding, carving was much more prominent (if you remember the old Burton PJ boards, F2 had their Speedster race boards, Raichle had their hard boots (which are now the Deeluxe) etc. And so I think carving never went out of style in Japan. There's a lot of groomed runs in Japan and while Japan/Hokkaido is now famous for powder and off piste, I think snowboard racing and carving has always had it's place. When you see how many Japanese manufacturers make carving boards (Moss, Ogasaka, BC Stream, Gray, Yonex, etc.) it's clear that the carve/race market is vibrant even with 1/3 the population of the US.
I ended up posting all of the pics to this thread, so no need to go to Imgur.