South Korean government did not support Koreans living in Japan. Whereas North Korean government even though isolated state were poor. They were 60 years ahead of South Korean
government regarding to helping Koreans living in Japan. This is basic reason why majority
of Koreans living in Japan went to North Korean government run schools. More then 80 percent
Koreans living in Japan were born in island province of " Chejudo". But more then half of them
affiliate themselves with North Korean schools. Main reason is North Korean government helped Koreans in Japan in very very tough time wherease South Korean government did nothing. Koreans living in Japan don't really like to divide themselves into North or South Korean political alliance. Majority of them consider themselves as Korean from Southern Korea but they had no choice to preserve Korean cultural identity majority of them went to North Korean government run schools.

If you combine Sakalin Island Korean population. Korean population in Japan probably would be
about 1 million. ( Korean, mixed Korean/Japanese, mixed Korean/Russian) populations.