Hmmmm, I can see several possible reasons:
1. Code of Bushido, and not wanting to appear like a defeatist, so wouldn't want to wear one – surely, the pilots won't be shot down, if they make up their minds to be victorious. Also gives them a bit more incentive to be aggressive, knowing they simply can bail out when desired.
2. Lack of supply/availability for the pilots.
3. Added weight to the plane, and might make it a bit less comfortable during the fight, distracting the pilot.
4. Perhaps they were expected to ram any enemy aircraft, vessels, vehicles, supply dumps, etc., in order to make the ultimate sacrafice – see item #1.
All of the above is conjecture on my part, but does fit with some of the stances they took during the war.
Not sure if it applies to their Air Force, but does to their pilots fighting American naval forces.