Convention agents are part of the problem, but most conventions hate dealing with them. The con would much rather make a deal directly with an actor than have to do everything third party through any kind of rep. Also, the rep will always ask for more perks and more money than the celeb will alone (since the agent will often want a free hotel room as well).
I think the big problem here is with DragonCon not taking the fans desires into consideration. The people in charge of booking the celebs may personally like Happy Days better than Aliens and that makes a huge difference as to who gets booked for shows. Unless there is a big fan outpouring, or the staff can look at another convention where these celebs showed up and the show had huge numbers of attendees, then there is little difference to them between Donny Most and Al Matthews. In fact, they would nine times out of ten choose Most over Matthews because Most has a chance of drawing in people who were fans of Happy Days, but who normally wouldn't go to a sci-fi convention, and that means more money. Aliens fans were going to DragonCon either way.
DragonCon has changed a lot over the last few years, and I myself am butting heads with them over the costume contests (they want the Miss SpatCave contest to be PG and more family oriented for some reason), and thier choices of celebrity guests (Star Trek Fan Film nerds?!?!?!).
I think it may be time for the US Aliens fans to step up and put together thier own show the way the UK fans did. I'm sure there are enough people who can pitch in one way or another to help launch it.
And as mentioned above, do whatever you can to get your voice heard by the convention so they know what the fans want.
-Spat
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