There's numerous reasons as to why you never see japanese players in Europe or in the Sh.
First, there is no true professional player in Japan. All the teams are company clubs and the players are employees, actually working in the company while training and playing rugby. They are destined to work in the company once they are finished playing. So going abroad would make it very difficult for them to get back in the system afterwards.
In terms of rugby, Japan is very much closer culturally to the Sh. European clubs and players are even less known than their SH counterparts, and that's saying something. However, it is extremely difficult to get a foreigner S14 spot in Nz or Aus (assuming you're good enough), and the money is just not good enough in SA or ITM cup compared to Japan.
And finally, Japan is an island, and it has very much the island mentality thing. Japanese are somehow social inepts outside of their little bubble, and many fear the outside world. If you think I'm joking, you ain't been here. They are much more confortable staying in their comfort zone. Japan is a risk-adverse society. Even in soccer the best players prefer to be the big fish in the small pond rather than go to Europe, realise they suck and warm the bench for 2 years.
On top of that, the rugby world in japan is very conservative, self-centered and completely deluded (not just the rugby). For example, the big thing here is University rugby, where everybody can watch and rejoice in glory about how good the teams are bla bla bla. And then they don't understand when they lose against Samoa B in Tokyo...
So, I rambled a bit but I hope you got the point...
