After WWII, America allied itself with Japan to combat communism in East Asia. With America behind Japan, it was harder to bring international pressure against the Japanese. While some of Japan's war time leaders were tried and hung for war crimes, Emperor Hirohito was not. He played a role in Japan's wartime agression and should be hung for the war criminal that he was, but unfortunately, he was not. The U.S. allowed him to remain as a ceromonial head of state. The Japanese constitution was redone and we have the de-militarized Japan that we know today. The U.S. also struck some sort of deal not to persecute the Japanese for any more war crimes in exchange for some top secret military information. The U.S. government received the details of the biological and chemical weapons used by the Japanese unit 731 against the Chinese in Manchuria.
Also, the communist government in China did not aggressively pursue reparations against the Japanese. After defeating the civil war in 1949, the tasks of keeping the Nationalists from regaining the Mainland and rebuilding the country kept the Communists occupied.
Whenever individuals sue the Japanese government for reparations, the Japanese point to some sort of treaty signed in 1962 or 1964 in San Francisco that absolved them from paying reparations.