Having been a part of this hobby now for 40 years (am I really that old?), I've seen lots of changes over time. (This is a long-ish read - proceed with caution!)
First, we had the people who got into it because they were fun (the 1970s and before).
Then, we had people get into the cards because there were valuable (the 80s) - and this brought on the jump in the number of baseball card shops all over the place. So for all of you talking about the 80s and 90s with the huge amount of shops, you have have that "boom" to thank for that.
Then, we saw the card companies almost self-destruct. The glut of sets and cards they unleashed on the market made all those kids who were collecting in the 80s walk away. (Many of us are back, but who here on this site who is over 40 hasn't said "I was out of the hobby for a few years there in the 90s/00s" - I don't think there's anyone who continued the buy/sell/trade aspect of their collections continuously. So, blame #1 - the companies for making products that caused people to walk away.
Additionally, during this time, I noticed that card shows and card shops were starting the trend of selling less cards and more memorabilia, be it jerseys, hats, autographs, or other non-card items. Maybe they were more profitable, maybe they were more popular, but this also helped move cards out of shops.
The other thing that has killed it is the change in distribution of packs of cards. I remember back in 1981 when Donruss & Fleer released their first baseball products, that packs were available at every convenience store, drug store, deli, and many grocery stores. Maybe I was just lucky, but we had a candy wholesaler near us in southern NJ who also carried baseball cards,which in turn meant the public could buy boxes at wholesale prices and that packs could be found in candy shops. They really were everywhere - and this was even before the over-production days! Now, Target, Wal-Mart, Meijer (which we don't have around me), and maybe a few other stores and of course card shops (which are few and far between now) are the only places selling packs & boxes. They are not in the convenience stores any more.
We had many card shops in southern NJ through the 80s (although I really only went to one) and we had a card show somewhere there almost every weekend. My dream as a kid back then was to have my own card shop. My best friend actually did achieve that dream - in at least 6 different locations (permanent stores, not flea markets, which he also did), 4 of which were in the 90s/00s. He recently had 2 more locations, and he just couldn't get the people coming into the store at the first of those 2 locations to keep the place open. It would up being an extremely expensive storage unit. So he closed that one down, and basically, the second one was merely an attempt to liquidate before moving from NJ to FL. But even then, again, no foot traffic. Maybe the interest level is just down. It wasn't due to pricing because he'd charge what he'd see stuff selling for online.
This is why the hobby looks like it does. This is why most people's hometowns arre now Hobby Wastelands.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).