Well I would assume because they have no interest in Japan? Or afraid of discrimination?
Debito Arudou - First American man known in history to live in Japan, create his own land, be allowed to enter a hot spring, because he fought for his rights (against discrimination towards foreigners), and to join the Japanese diet (government kind of thing). The discrimination has lessened now, but it was really bad back in '95.
http://www.debito.org/