It is not because the US doesn’t lock people up enough.
Once again, I don't know much about Japan but I doubt there is a large ethnic minority that is only about fifteen percent of the population but somehow accounts for almost half of all incarcerated people in Japan.
From Google:
> Low Incarceration Rates: Japan maintains one of the lowest incarceration rates in the developed world (roughly 35 to 40 prisoners per 100,000 residents, compared to over 500 per 100,000 in the US).