Why aren't there many billionaires in Japan?

Question:

For a country that has such a high GDP, why are so there few billionaires?

Answer:

I don't know how come you think "not so many". There are 23 billionaires in 2010 Forbes list.

Actually, billionaires in Japan and in the world have much of their asset as stocks. But as Japanese stock market is slumping in the last 20 years, their asset value has been falling.

There used to be more than 100 of them in Japan. But now it's fewer because of stagnant stock market.

Other stock markets are doing fairly well.

Addition:
>because Hong Kong alone has nearly double the country of Japan's billionaires

HK has very low income tax rate. So billionaires want to move to HK. This place has completely different reason as to why there are many billionaires.