It has everything to do the Zero's flaws, IJN carrier doctrine evolved around many of the inadequacies of their equipment or lack there off, the Zero, re it's light armament was one of them and that directly lead, combined with other problems such as poor damage control to the carriers being lost, that's a fatal flaw. The Japanese lost experienced pilots very early on in the war because their aircraft in general had a low survival rate after being hit, the air forces of all the other fighting nations put great emphasis on pilot survival even to the detriment of the aircraft's performance, the Zero gained it's agility by not being protected or protecting it's pilot, that's a fatal flaw. As to your comment on the Zero not catching fire easily, there's hours of gun camera footage/film of Japanese planes literally bursting into flames when hit.