Hirohito became a figure head.
Here is the thing, he was always a figurehead. As had every Emperor from 1192 to the present day, with the exception of one.
The only exception is known as "Emperor Meiji", in what is known as the "Meiji Restoration". For the first time in almost 700 years the Emperor dismissed the Shogun and in essentially a "Self-Coup" assembled an army and fought a war to force the Shogun, the Samurai, and all others to place himself as a true Emperor in the modern sense and took total control of the nation. Whenever somebody watches "The Last Samurai", realize that the "hero" is actually fighting to try and keep the Shogun in power. A dictator who wanted to keep Japan locked in a backwards feudal state that Europe had left centuries before.
However, that the Meiji Restoration was not to last, only from 1868-1912. When Meiji died his son and heir was Emperor Taisho, who was literally mentally impaired (most likely caused by childhood meningitis). And as he was not able to rule himself as his father did, a new cabal arose to rule in his name. But unlike the Shogunate which had become hereditary positions, this time it was high ranking military officers and retired military officers that took control.
And when Emperor Taisho died in 1926 and his son Emperor Showa took the throne, that same cabal of officers remained in command of the country. So just as it had been for 700 years before his Grandfather, Japan was once again a nation that was in reality ruled by an elite group but this time career military officers and not a hereditary nobility. That is where the Imperial Rule Assistance Association (essentially the Japanese Fascist Party) came about to "assist" the Emperor in ruling the nation. And the "Big Six", the actual leaders of the nation during WWII. All both the leaders of the Imperial Rule Assistance Association as well as high ranking military officers except for one.
Prime Minister: Admiral Suzuki
Minister of Foreign Affairs: Shigenori Togo (the only civilian on the council)
Minister of the Army: General Anami
Minister of the Navy: Admiral Yonai
Chief of Army General Staff: General Umezu
Chief of Navy General Staff: Admiral Oikawa (replaced by Admiral Toyoda in May 1945)
Those six were the ones that really ran the nation. Emperor Showa attended the meetings, but he literally was kept out of sight behind a screen. He was allowed to listen out of sight, and prohibited to ever say anything in those meetings. So it was not that he "became a figurehead", he was a figure head from the day he took the Chrysanthemum Throne. Just as his father was before him, and every Emperor going back over 700 years with the single exception of his grandfather.
In essence, after WWII Emperor Showa was essentially placed in the position of the contemporary English monarch. They reigned, but did not rule. However, unlike all the previous Emperors other than his grandfather, he was now allowed to speak to the Japanese people (once he actually learned "real" Japanese - he actually did not speak Japanese but an archaic court version that had not been used in centuries). And as such he spent the rest of his life trying to lead his people, but never ruling them. And it was a role that apparently suited him well.