Yes and no.
I haven't been to one in about 4 years for financial reasons, but I have noticed a few things which could explain the decline. The internet is obviously the biggest factor.
1) Age. Most vendor's average age is about 55. Meaning, the younger crowd hasn't picked this up.
2) Ripoffs. Too many guys think they can pull a fast one. $10 timing light that doesn't work for example. If you were a first time go-er, would you go back after that experience? Unfortunately this is becoming the norm, and the younger crowd isn't as careful and a bit naïve to avoid scams like this.
3) Too many rules. It seems that every year, there are about 10 "new rules" added to the seemingly endless list of rules. The vendor's rule's are probably 3 times the size of customer rules. Each year someone tries to pull a fast one on "the system" so the organizers have to add rules to prevent damage, keep up safety, and be allowed back the next year. But if you have to fill out a dictionary of forms just to have a sale where you might make a couple of hundred bucks, are you gonna bother? Back when everyone was in the spirit of things, you didn't need so many rules. But today everyone goes to find a steal, or make a fortune, and you cant have both of those every time. As a result, you are seeing a shift in vendors, and instead of some regular joe cleaning out his garage and firesaling everything, you have larger vendors coming in that can support the cost and hassle. Swap meets these days look more like a shopping mall than a garage sale.
4) The wrong kind of advertising. Around here, we have 2 kinds of swap meets. The public kind that are heavily advertised and usually tied to a county fair. And the SUPER SECRET kind where they are made to happen by organizers, but you don't really know about them unless you were specially invited. This is the same comparison as say a "show and shine" in your city, or the "PRI" show. The PRI show has less fluff, its industry specific and you get right to the meat and potatoes. Well the SUPER SECRET meets seem to have the better scores, and rarer stuff, where as the bigger more public meets seem to be a split between a regular flea market and a swap meet. But to keep up revenue, the secret meets are disappearing and too many people just out to enjoy the weather are showing up. As a result you get more "regular stuff" that you can buy anywhere, and less spaces for your regular joe that is trying to sell his all original garage kept Plymouth superbird with front end collision for $800 cuz hes too old to fix it.
BUT....
Just because they are in decline, doesn't mean they are on their way out. I think they just have to compete with the internet now. Swap meets still offer something the internet cant give you. I still find some kick *ss deals at them, that im glad aren't on the internet and I hope it stays that way.
1) CASH IN HAND. People are pretty convincing on the internet but flake easily these days. Nothing beats bartering when the seller can see/touch the cash, and the buyer can see/touch the item.
2) No hassle buy/sell for big items. Hoods, engines, body parts, wheels are all a gigantic hassle for the regular joe to ship. You gotta find packaging, you gotta get quotes, you gotta haul stuff around. Its not easy. At a meet, you bring what you bring, the seller takes it from you, and you get cash. No hassle, buy/sell whatever you want and carrying it away is the buyer's problem.
3) Unexpected deals. I have stopped to talk to random sellers at times because I recognized an item from a rare car or something similar, and ended up selling him something I had at home that I never considered selling. Made a few bucks that day. Hard to do that on the internet.
Theres ups and downs. I don't think change is always a downer. I still think they are mostly positive.