Mainly because when ever I get into a battle with any other class as a Pyro the effect of the flamage kills the enemy about 30 seconds after they have already creamed me with what ever weapon they were using.
Yes, the Pyro continues to deliver damage after hitting the enemy but what's the point if they kill you as well. For an offensive class player I would expect you to be able to continue advancing after the initial contact with the enemy. If he was re-categorised to Defence then I could understand a little bit but currently he's useless.
Also it's not like one hit with the Pyro will continue to depleate your health till you reach zero, it stops before you die unless there is maintained contact or you are already low on health before the encounter.
Panzer