Hopefully, more people will contribute and expand on this but there defintiely was an aequivalent to the Nuremberg Trials in Japan called International Military Tribunal for the Far East or Tokyo Trials, FYI.

The premise of the question is IMHO not correct. High ranking officials and commanders were tried and convicted.

At the International Military Tribunal for the Far East (1946-48) twenty-eight military and political leaders of Imperial Japan were tried for war crimes. Six were sentenced to death, among them former prime minister Koki Hirota, former war minister Seishirō Itagaki, General Heitarō Kimura (commander of the Burma Area Army), General Iwane Matsui (commander of the Central China Area Army) and General Hideki Tōjō ( commander of the Kwantung Army and former prime minister). Sixteen other officials were sentenced to life imprisonment, among them Okinori Kaya (Minister of Finance), Admiral Shigetarō Shimada (naval minister) and Naoki Hoshino (Chief Cabinet Secretary).

The emperor wasn't tried for several reasons. One was practical, he was deemed to be an important stabilizing factor and it was feared that the US occupation would become chaotic if the emperor was removed. It was also unclear exactly what his role had been. Compared to western monarchs the emperors of Japan has traditionally had a much more subdued role and acted as a figurehead, leaving state officials to decide on policy. That is controversial however and there is still not a consensus on Hirohitos exact role and power before and during World War II.