I wasn't a part of the process that brought this change, so bear with me.
In the context of minigun stutter shooting, it was more abuseable than any other "unlimited" ammunition weapon. Pyro stutter shooting isn't really that useful except for spy checking more efficiently; it doesn't affect its gameplay that much. Overweights that abused stutter shooting maintained a lot more mobility, which makes them a much bigger midrange threat, and does not deplete any ammo for when they have to hold off enemies.