If the bird population is low, or trying to rebound, a great many more birds can be killed with a single roundball than would be with legal shot size. Making it harder to kill them gives them a fighting chance to bounce their numbers back where they are thin.
The fatbird population around my area have been in decline for several years now.
As of the 2025 season, the legalities for my area are:
Scattergun only, 10ga max, No.2 shot max
These regs are always subject to change. This is the first year that we have had an "Alternative weapons" deer season, and the first year that buckshot for deer (No.4 minimum) is being allowed again in quite some time. After speaking with a few folks, and reading some, I think there has been an explosion in the deer population, and that is trying to be controlled.
I doubt we will ever be able to hunt a turkey, legally, with a single round ball around here, but I would like to know where it is legal.