Some of that is probably a factor of the room size. AC has gotten so hugemongous that the dealer room now makes up the better part of an airplane hangar, and there's just so much STUFF that sales are really diffused. If Joe Furry goes to every con with $100 to spend in the dealer room, it's pretty straightforward math to figure out that 200 dealers are going to make less as individuals than 75 will.
In theory, if 200 Joe Furries go to the con instead of 75, that should compensate -- but it doesn't really. Instead what you get is a Hollywoodization effect: the really hot items make a ton of money, and the second-or-lower tier items barely sell at all. It's not any kind of sinister forces at work, it's just human (or furry) nature. Popular items become more popular as they get talked about and showed off. Take Lion Of the Sun for example. He makes really intricate, high-end realistic fursuit heads which are very striking -- and correspondingly expensive. But his work generates so much buzz that he sold out on Saturday.
Or take another example, cooner, whose Lollerwear shirts flew off our table like they had wings. I don't have hard numbers yet, but I would guess he sold five shirts for every one of mine, because they were a hot item with a lot of generic appeal (and got a lot of buzz), whereas the Kitten Kaboodle shirt, as cool as it is (and I think it's pretty cool myself) is really an item specifically for Suburban Jungle fans. If the Lollerwear shirts had been, say, Buffalo Wings shirts, they probably wouldn't have moved anywhere near as fast -- not to denigrate BW, it's a great comic. But that's how fandom works, purchases are tightly tied into "self image". If somebody doesn't think of themselves as "a Buffalo Wings fan" they're not going to buy a Buffalo Wings shirt no matter how cool or funny it is, whereas the same core gag, genericized, can be adopted by anybody. My biggest single seller in terms of merchandise is the "Proud to Be a Furry" button -- same deal.
So why do I keep making SJ merchandise specifically, when generic stuff is where the action is? Well, a couple of reasons. First off, I have a hard time "thinking generically". My writing and art is character-oriented, and my merchandising ideas are the same way. I have a neat idea for a Brody Coyote coffee mug for instance -- which could be made into a generic gag with some effort, but is not how I came up with it.
Second is ego, really. When people buy SJ stuff, I can pretty much be sure it's because they like Suburban Jungle, and that's a great little ego boost. :)
I'm running out of room in this post for the rest of the topic, which is the "What Have You Done For Me Lately?" aspect -- so I'll do another post about that later.