Japan has a thriving freeski culture that is growing very quickly (but snowboarding is still dominant). Because Japan has very well funded mogul teams (the women, at least, are top ranked and have won Olympic golds) there is a pipeline of talent. Japan also has great trainng facilities in the form of indoor parks in summer, commercial water ramps all over the country, and trampoline gyms dedicated to skiing and snowboarding. Japanese skiers are not as well known as their talent would suggest they should be for the obvious reason of culture and language. Have you noticed that most of the big Euro freeskiers, like Candide and Jon, speak english?
That said, there isn't any Japanese skier at the pinnacle of freeskiing like THall or Jon partly because, as was mentioned, Japan doesn't have huge pro-park facilities. I also think that the Japanese have a harder time with expressive style sports due a cultural focus on refining the perfect technique that can often overshadow creativity. This is only my humble opinion but I have had a lot of experience with this in my younger days as a semi-pro fighter training in Japan.
That said, with freeskiing exloding over here, I wouldn't be surprised to see a lot more Japanese on podiums in freesking in the future. Especially Japanese girls coming out of Japan's thriving mogul scene.