12 gauge is THE shotgun gauge.
It stands right in the 'golden section', balancing the human capacity of manipulating objects and withstanding recoil with the necessary weight and velocity of projectiles necessary for killing flying objects.
I can believe that a niche market may at some point be overfed with supply of 12 bores so that there is a perceived overabundance of 12 and unsufficient demand for smaller bores, creating the 'no market for 12 gauge'.
I can't believe that this is a continious trend that will not reverse as the demand for smaller bores is satisfied.

Russian Hunting Shotguns