probably because the 10 gauge is fairly specialized any more, and a lot more expensive. kind of hard selling a $1500 12lb gun when everyone wants lighter and lighter shotguns that will pound you into the dirt with 3.5" shells. not to mention that a 12 gauge load with a similar payload and velocity to a 10 gauge load runs about 10 bucks a box less, at least locally. my costs will drop when i get around to buying a reloading setup.
the 12 is a lot more versatile, but i love shooting my 10 gauge. it patterns wonderfully, and is a real joy to shoot because the heavy weight soaks up the recoil.