I think the reason is clear: Instagram.
Many of the "newer" generation of comic book collectors are on IG, and as far as I can tell, very few of them ever post on the boards, let alone know about the boards, as far as I can tell. At Terry's show last January 2020 (how very long ago), I met a bunch of younger collectors, dealers, and "investment-focused" folks with a passion for GA comics, and they convinced me to join IG. Now as a user, I can see how a lot of the appeal of IG is simply that it is SO EASY to access and use. With one tap on your iPhone, you have direct access to all of the latest posts, without the nuisance involved with the CGC boards site of logging in, combing through the threads, scrolling through the threads.
The simplicity of use, and the quick consumption of content on IG, holds a lot of appeal. Waiting for the microwave to finish warming up your food? Just click IG! Finding your Zoom meeting has gotten really monotonous, just click IG! A gratifying 20 seconds of distraction.
Paradoxically, any given post on IG tends to elicit at least a couple interesting response comments, whereas on these boards, as soon as the "LIKE" button appeared following the CGC Boards upgrade, it seems most people are now just content to respond with a "LIKE" rather than engage in any discussion. It's nice to collect a lot of "LIKES" on the boards, but I think it has created a culture that's a bit less engaged.
The good news is there really does seem to be a large community of newer (younger) collectors who will help keep the hobby afloat into our retirement years. The bad news is that IG and other online one-tap sites have diverted more and more of the comic book community away from the boards. There are a relatively small number of stalwart boardies who post here and on IG, and I tend to gravitate towards their posts on IG.
Maybe other boardies here can pipe in on their experiences with IG.