Why wasn't Hirohito tried at the end of WWII?

MacArthur understood that if he put the emperor on trial he'd risk starting another war. The Japanese at the time regarded the emperor as divine, and would literally fight to defend the concept of "emperor as divine leader of the nation". MacArthur was sufficiently pragmatic to accept that leaving the emperor in place as a symbol of Japan was necessary to keep the Japanese people from revolting violently against the outnumbered US occupation forces, who I suspect were virulently hated by the Japanese.

This is not an "answer" exactly, but consider it American folklore... it's how I understood it from my family who lived through and fought in WWII -

Hirohito was considered an actual god, not a man, by the Japanese. When he surrendered on the radio it was the first time the Japanese had ever heard the voice of their living god. They capitulated immediately. Although he hadn't made the decision to bomb Pearl Harbor or even go to war with America, he had been used by the Japanese generals as a divine cause. It was necessary for America to not kill him or martyr him, but to flip him and use him to un-brainwash the Japanese who would have fought to the last man.

The same kind of thing happened with the "de-nazification" of Germany. At that time, America had the money and was willing to spend what would now be billions of dollars to rebuild foreign countries we had just demolished as long as they "agreed" to accept the world order we were trying to build. (No particular judgment there -- the world order would have been worse under the nazis). And since we had just conquered them and were sending tons of money, they had no reason not to agree. The same people in Germany and many in Japan remained in power, but they were now America's tools. And this worked out pretty well for the Japanese and the Germans. Not as great for the Americans, but the impulse to buy out the remaining leaders, rather than martyr them, was probably the right move.

In contrast, we won the official war in Iraq back in 2002-3 but failed to give them their local figureheads back, and we ended up with a 20 year guerrilla war and with ISIS, and withdrawing from there and letting Iran and Russia handle it. So basically, we were smarter at the end of WWII about how to use former leaders to redistribute power, under our auspices and control, and with the help of our money.

Remember Hirohito was regarded as "divine" so keeping him around to instruct the Japanese civilian and military population was seen as critical in retaining control without an enormous military occupation force.

Hypothetically, his execution, trial, or even his arrest could have triggered an unstoppable wave of rioting, where nothing short of genocide could have stopped the attacks.

It took the God Emperor to say "stop fighting" to stop the war and then he was more useful as an ally than as a martyr.

It was partially MacArthur's idea. I suspect that, aside from considerations regarding internal Japanese stability the swiftly developing Cold War (things soured as soon as Germany surrendered) motivated the idea to not rock the boat overmuch. Remember that there was a sizable Japanese Communist Party in the 30s.

Also Japan really has had, as odd as it may seem to us, the notion of absolute-power, divine, figurehead emperors. Prior to the Tokugawas, the emperor would not really rule until he abdicated to let his son take over. At that point he would assume real power, over a decentralized feudal system, while his son performed rituals.

From wikipedia:

The role of the Emperor of Japan has historically alternated between a largely ceremonial symbolic role and that of an actual imperial ruler. Since the establishment of the first shogunate in 1199, the Emperors of Japan have rarely taken on a role as supreme battlefield commander, unlike many Western monarchs. Japanese emperors have nearly always been controlled by external political forces, to varying degrees.

Later on, the Shogunate took over and put the emperor even more on the back burner while he was nominally in charge. If I recall correctly, in later Shogunate, the Shogun himself became a figurehead while there was another influential group doing the real work.

Not sure how the 1868 Meiji Restoration modified things, but certainly the military ruled the governmental roost, having a defacto veto over cabinets.

So while it was a convenient and calculated whitewash not to get rid of Hirohito, the idea of him not totally having been in charge isn't merely a fiction and the extent of his actual decision and control is still being debated.

Because of fears of instability and communism spreading to the area. Treaties ended up being signed to turn it into more of a democracy which was more in line with what the US wanted.

https://www.historynet.com/ask-mhq-hirohito.htm

American policy toward Japan from August 1945 was dominated by fears of the communist strategic threat to Asia. It is sometimes suggested that Gen. Douglas MacArthur, as the Allies’ supreme commander, imposed his personal will to spare Hirohito. MacArthur certainly acted autocratically, but the contemporary documentation shows that Washington’s view marched with his own in rating the stabilization of Japan above all other considerations.

If every man guilty of war crimes had been punished in strict accordance with law—in Germany as well as Japan—hundreds of thousands of executions would have been carried out. Nobody had the stomach for these. With the Red Army in Manchuria and America’s protégé, Chiang Kai-shek, struggling for control of China, the United States was concerned about the overall volatility of the region and opted for a light-touch occupation of Japan.

The Japanese showed themselves almost slavishly eager to conform to their conqueror’s wishes, and indeed displayed an amazing enthusiasm for all things American. The emperor and the nation’s political leaders swiftly accepted a draft for a new Japanese democratic constitution produced by MacArthur’s staff. On January 1, 1946, Hirohito issued a proclamation denying his own divinity and denouncing “radical tendencies” among his people.

MacArthur publicly applauded, commending the emperor’s brave decision to take a “stand for the future along liberal lines.” After that, it became implausible for the emperor to face war crimes charges. Only a few hundred Japanese were tried, to satisfy American public opinion. Most notably Gen. Hideki Tojo, prime minister when the Pearl Harbor attack was launched, was hanged, along with Tomoyuki Yamashita, who commanded in Malaya and later in the Philippines. Gen. Masaharu Homma was shot by firing squad, after being convicted of responsibility for the 1942 Bataan Death March.