There's a population difference. While the US is more than twice as populous as Japan, it is less than a fourth as populous as China. It would be crazy if Japan became wealthier than the US; it would be only natural if China did.
There are also many other differences: differences in resource/energy availability, differences in the global environment, differences in policy options available to the government, differences in internal structure and diversity, etc. The Japan-China comparison is popular these days but it misses a lot of the things that distinguish China from the old Japan.
