My entirely unscientific take:
I collected FB as a kid, stopping around 1975. That is the only reason I collect it now, and all I collect are the cards from 1979 and earlier, with rare exceptions.
I know quite a few people who are alienated from the modern NFL for a variety of reasons: thug players; concussion denial; the injuries the sport inflicts on its players; poor treatment of its cheerleaders as workers...the list just goes on.
I find the sport boring and slow, especially when televised. The actual play takes up only a small fraction of the time of a telecast. I cannot sit through it.
The 1960s-1970s had such fun players. We don't see that any more what with all the personal branding considerations. Now, that is endemic to all sports, but it is such a contrast from the days of these guys:
