Dragon*Con is very expensive, very far from the main HQ (In Seattle), and stuck right next to Gen Con and Pax Prime (Gen Con is 2 weeks before it, Pax Prime is the same weekend). It's also, from what I've heard, not a great "vending" convention, as most attendees are going for the spectacle and panels and costume stuff, not to buy stuff. I know CGL would love to be able to do the show, it just doesn't end up feasible, especially since they're attending Pax this year (Which is in Seattle).
Bull