I think part of the reason is that C cells are relatively expensive; most of the time when I see an 8-pack, they're the same price as D cells that have a significantly higher capacity.
I would like to see some 3XAA lights put in a 1D format. It wouldn't be intended for EDC, but it wouldn't have to boost voltage and runtime would last longer than 1xAA or 2xAA.
If I had a 2C light, I'd just get those cheap adapters to stick 4xAA NiMH's in.