The occupation authorities were focused on rebuilding Japan enough that it would become a bulwark against Soviet expansion. Anything that would demoralize or anger the population, like trying the Emperor for his part in the war or having trials related to Unit 731, was not going to happen.

Though even in that light it's difficult to understand why people like Ishii weren't quietly hanged. I can't imagine his cooperation produced a nontrivial amount of useful information we couldn't have gleaned from documentation.

Similarly, cold war geopolitics quickly overcame the allied effort at de-nazification even while the Nuremberg tribunals were happening. Many prison sentences were commuted by ~1950. West German cooperation was more important.