The 12 gauge shotgun shell is problematic when it comes to feeding out of a magazine, as mentioned by others already. A flat nose, a rim, no taper or bottleneck to help guide the shell into the chamber makes for clunky feeding. Plus, the large size of a 12 gauge shell makes a detachable magazine personally unappealing. Who wants a foot long magazine sticking out of their weapon that gives them ten shots?
Now, Kel Tec came up with a novel idea with dual magazine tubes and a neatly laid out bullpub design. I don't know how that will prove out in use, but certainly it blows up the idea that shotgun innovation has stagnated.
But I have a limited imagination, as I can't think of a scenario where an Ithaca Model 37 riot gun (with or without an extended tube) would be found wanting, or the same gun with extended tube, heat shield, and a mounted light wouldn't be close to ideal.