We LOVE Worldcon....but here's what happened...
I can speak on behalf of most of the Professional Artists I know, and it comes down to costs.
We love the fans, the Worldcon, which we realize takes A LOT of work to put on.
Back in the 1980s, it was commonplace for us Pro Artists to schlep or ship our work to the convention. The 80s was a great time, SF looked good, major authors were doing major works, the covers were the best they'd ever been. Costs were low. Even in the 90s it was still viable. I can remember in 1996 shipping 3 large boxes of artwork to the LACon of that year in Anaheim. It was a lot of fun, I won a Hugo in fact. The boxes cost me something like $300.00 each way for a total of $600 and change. I made something like $4500 in the show, so including everything, I still made money.
Occasionally if a Worldcon was close to drive to, I'd do that too. Rent a larger car and bring down the works that way. In 2004, the Boston Worldcon was but 50 odd miles away. I still had to rent a larger vehicle and bring all that work there, the hotel was a lot more in cost, then add to that the parking of said vehicle. Fortunately I made something like $5000 and change in the show from sales. But I found I spent close to $3400 in costs overall. Prices were going up.
I did a few more after that, including the Japanese Worldcon in 2007 where all the planets lined up in terms of costs.We got a free frequent-flyer trip for two on Continental Airlines. We went through a Korean travel bureau, their version of Priceline, in effect and got a convention hotel at MUCH LESS than the convention rate. I brought all my art for the tiny artshow(small paintings!), as hand-carry on luggage. I sold every single painting-the Japanese love to buy US artists' work- and it more than paid for the trip. It was actually CHEAPER for me to go to Japan for a Worldcon than it was for me to go to Boston for one, 50 miles away.
More and more, Federal Express and other reliable carriers have increased their rates, box size prices and so on. And more and more, the return in terms of sales at the convention has been less and less. Thanks to luggage inspections at airports, "bringing art with me" has become a risky and reckless idea. The work is un-insurable.
In 2010, the cover story was a "vague" Australian tax law(which may or may not have been applicable if the show's sales topped $50,000 AUD which was unlikely) on non-profit events such as the Australian Worldcon, but the reality was a lack of manpower. The upshot negated any formal artshow from being held. Instead an "art display" 80% of which was from the collection of a Sydney-based collector who graciously lent them the works he purchased over the years so there was something to look at and not a bunch of blank panels. This collector also spent much of his time, sitting with and looking after his valuable collection as there was no security to speak of, nor was there as I was told, insurance. I didn't blame him for largely spending his time there. Also, some artists(including AGoH Shaun Tan) opted to put up and sell their own works much in the same way dealers might sell their wares, cash and carry basically.
It's the shipping costs that it all comes down to vs the return in sales that are not always congruent. So while people ask "What happened to all the name artists?"....it's simply cost that we can't do this anymore. My personal view is also that, Worldcon has changed and few people are interested in the physical art like they used to be, with all the interest in digital media. And it has become a lot of work to prepare for these events. My memories are long and I will always remember the good times, but, they've passed. I see a future of an artshow-less Worldcon, due to insurance costs and lack of manpower and, as digital art becomes the mainstay, a lack of physical art.