Here are some things I can think of off the top of my head, plus maybe some rambling
The collector base has changed A LOT. Actually, let's re-phrase that to purchasing base. There are a lot more people doing case busts looking only for "hits" of cards that will sell well. They aren't too interested in hits of players that don't sell well. This is one factor that changes the "hobby". For these purchasers, instead of the hobby being focused more towards a collector for the love of sports, cards, and/or players, their hobby is making money and "flipping" the cards all while making videos of them opening it.
Card shops have to pay bills. They will sell anything related to the industry to do so, i.e. gaming cards, etc... This is probably why it's hard to recognize a card store when comparing to previous years.
There isn't nearly as much variety as before. Panini pretty much does all Basketball and Football, Topps does Baseball, and Upper Deck does Hockey. Of course there are exceptions for certain products but that's the basic. Remember when Topps did all sports? However, they also didn't have nearly as many sets. I used to love Donruss, even with all of their inserts because it was competition with Topps and each had to come out with a better product than someone else. Now, Topps simply has a range of least expensive to very expensive and they make products for all price ranges in between. For example, look at the range of products from Topps Opening Day/Topps base all the way to Five Star.
Anybody who says this is supposed to be for kids doesn't quite know what they are talking about, especially taking into consideration the cost and current technology trends. Today most kids would rather have the latest tech gadget so that they can compare their lives with their friends (facebook and you tube). Trading and collecting cards is kind of a lost hobby because with the advances in technology there is alot more stuff available to pass the time. When I grew up in the 80's kids collected and traded cards as part of their things to do simply becuase technology wasn't available. Trust me, if iPads were out in the 80's I think the trading card business would be in a lot worse shape than it is now. Imagine playing Donkey Kong on a portable device with a touch screen! That would have taken up a lot more of kid's time so there would be much interest in buying packs.
On top of that, the hobby and the idea of collecting something has to be learned from somwhere, usually parents. As the kids who grew up with technology that required no need to pass the time in other ways, they simply won't have the memory of collecting and trading sports cards so they won't pass it on. I honestly believe that this hobby is becoming more and more narrowed to a very specific group of people as time passes.
But as theplasticman above said, where does one begin? There are a ton of variables to consider when trying to figure out what happened to the hobby. These were just a couple of my opinions and viewpoints.
