There must not be much of a demand for them at that size. One of the LHBS that I frequent sells a ton of the 5 and 6 gallon, non-ported ones mostly to wine makers and homebrewers who use kits, both of whom probably don't go above a 5 gallon batch.
Just a guess.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein