Oops! Yes, ancient question!
I would guess that a 20 inch barreled shotgun was impractical for everyone except stagecoach guards, so why would any company make them for such a small subset?
I'm guessing (totally guessing) that shotguns were like unhemmed pants. Buy them long and cut to fit! With a "coach gun" you don't care if the bead is cut off with the longer barrel!
My 2 cents (2013 cents, not 1860 cents!)
Oh and yes, my impulse buy of the Chinese double is based on the fact that it was on sale, it was in the store where I bought my Rossi 92, and I didn't want to pay another $25 fee for California paperwork elsewhere (you can put multiple long guns on one DROS form if you buy them at the same time). And it looked decent. I'm sure if I get into this game, I'd move to something else, but I'm already half broke from the Rugers, Rossi, leather, hat and boots! So ... cheaper functional shotgun sounded good to me!