Should have executed Hirohito, otherwise did fine.
The U.S. wanted Japan to become reconciled to its defeat and to become an American ally. Executing Hirohito would have been possibly the worst thing it could have done as far as sabotaging these goals.
This argument reminds me of the "We should have executed Jefferson Davis" claim on another thread.
I'm not entirely sure Japan
has been reconciled to it's defeat in World War II, and a lot of that had to do with the fact Hirohito was left alive and on the throne.