I think there is also traveling and hotel expense to consider, especially when they're just gotten back from GenCon AND most of the senior people live in Washington State....
Also, DragonCon is a wide-open con in which gaming is only a part of the con. There's sci-fiction, fantasy, cosplay, (Much more than GenCon), anime, and a heck of a lot more celebs than GenCon. GenCon had 30k gamers, people who CGL might be able to see a game to. While DragonCon may have 30k attendees, most won't have an interest in gaming. I think CGL thinks that it isn't cost effective for them to have a D*C presence....
Craig