Why would he have done that?

Edit: Let me be clear, there is no reason for a Japanese Emperor who lost a war to kill himself. There is no reason for a lord who lost a war to kill himself either. (I'm sorry for the somehow aggressive answer, I hit save a bit too fast.)

Reports of Japanese people committing suicide due to loss of honor have been greatly exaggerated. Seppuku or hara kiri existed, but was not a regular practice. There were a lot of Japanese people who committed suicide during the final weeks of the war, particularly in Okinawa and nearby islands, but that was because the US was going to occupy their island and they had heard tales of horror and were terrified of torture and rape, which they were sure were going to happen (but didn't, except for rape in scattered instances).

In any case, Hirohito was above the fray--infallible and not to blame for the defeat.