Japan's peak of power was in the 1940's before it was bashed into submission by the Allies. Even though Japan's economy grew rapidly from the 50's to the 80's, militarily and diplomatically it was shackled to the US by virtue of 50k US troops on Japan's soil. As long as these troops remained on Japanese soil, Japan could not become a power militarily or diplomatically.
Furthermore, in history Japan only became a great power in the 19th and 20th centuries. Apart from a failed invasion of Korea during the 16th century, Japan otherwise tended to be an isolationist, inward looking country. It should come as no surprise then that the current Japan is converging back to this state in the next 30 years.
In contrast, China has been the top dog of Asia for a very, very long time. It's weakness during the 19th and 20th century is an anomaly, not the norm. We shouldn't be surprised that China is converging back to it's status before the industrial revolution: top dog of Asia.