Someone promoted this game in some popular forums in HK and kick started it. In HK many young people watch NBA and play street basketball. The interest is there and it was simply a matter of giving the push that it needs.
I know there was a very popular Japanese comics (definitely in the top 10 in their all time selling record) about basketball. So it's not as if basketball is alien to them. With their large internet base there is no reason why Japan should be less successful than HK.
You are talking about the Japanese manga "Slam Dunk". Definitely a popular magazine, and whilst Basketball isn't as popular as soccer or baseball, it definitely has a following. Even if just a very small percentage of that group played/stayed, then Japan could easily number members in the 100s.
Does someone know the approximate number of different Japanese users who played this game?
Is it a case of a game not being publish good enough, or is it a case of users leaving the game?
I would guess (from a brief perusal) about 5-8 of the supposed 35 users are Japanese nationals. The vast majority are content to just 'play' and not participate in community as the majority of that is in English.
As far as your second question goes: both. It's not so much a case of the game "not being promoted well enough", but "not being promoted at all".
And at the heart of this thread: new users get stuck in leagues which nigh on but guarantee they will have a tough time of it in their first weeks (getting placed in the strongest divs in the Nation), and are more likely to quit as a result. Even if this were changed (either coming up with a better system of promotion, and/or developing a more equitable way of new-team-placement), it'd help retain new teams.
http://with-malice.com/ - The half-crazed ramblings of a Lakers fanatic in Japan