This is my first time answering a question on this sub, apologies in advance if what I write does not conform to the sub’s rules!

For reference, I am a historical sociologist who works on the 20th century, but I primarily study college admissions and economic policy. So this topic is not quite in my professional wheelhouse. But I was also raised playing bridge and was, for a few years, the youngest life master (a title awarded for earning a certain number of master points at American Contract Bridge League tournaments). My father is a professional bridge player (something one can still make money at, albeit barely) and my whole life I’ve been fascinated by this question. At age 10, I was routinely the youngest player at a tournament by 20-30 years. At age 37, I am often the youngest person in the room by 20-30 years. The decline is very real and a matter of much consternation to serious players and the league. As such, I’ve looked into the scholarship on the topic every few years since I started graduate school. And as far as I can tell, there is not a definitive answer, though there are various plausible theories.

The first thing worth saying is that the rise of bridge may be more surprising than its decline. Bridge is an enormously complicated game. As this 2007 New Yorker article on its decline notes, while you can learn the rules of chess or poker (or hearts or spades) in half an hour, learning to play bridge takes far longer, especially with modern bidding conventions. Article: https://www.newyorker.com/magazine/2007/09/17/turning-tricks

So the tremendous rise in popularity of bridge in the 1930s-1940s remains to me as puzzling a question as its decline in the 1960s-1970s.

Given all that, the sources of the decline usually cited are changes in how people socialized, including the rise of TV, and various changes in gender norms (the emergence of co-ed dorms, casual dating, etc) such that hanging out as two couples declined as a super common form of socializing. These changes, and I don’t know that anyone really knows which mattered most, led to network effects - in the 1940s, you learned bridge because everyone learned it, you could always find a game, people expected you to know, etc - and once that started unraveling, the process was quick and dramatic. While participation in serious/competitive bridge continued at high levels for some time after (as those who learned the game seriously in their teens continued playing throughout their lives), new entry into the game dropped precipitously such that by the 1980s, vanishingly few children played (fun fact: the “junior” category in bridge goes through 26). And now that those who learned in the 1930s-1950s are beginning to pass away or become unable to play, tournament participation has also declined precipitously.

I’ll try to add some more sources when I’m next able (have to go mind a toddler now), and I am also very hopeful that others may chime in with more serious scholarship that I might have missed!

One relevant source: https://www.tandfonline.com/doi/abs/10.1080/01490409109513148

This is a fascinating question, OP. As somewhat of a follow up, when did Bridge hands or plays or whatever they are called (I don't really know anything about the game, but I'm happy to see this question because I'm curious and eager to learn) start appearing in newspapers and what is their purpose? Do people play Bridge by correspondence like they do chess or something like that?

GenX here and this is an observation not an answer. Aside from my silent generation dad and his poker buddies my parents never played cards. In my early work years I had a windows computer but no internet. Those computers had games such as bridge or hearts and I taught myself how to play but none of my peers played and I lost those skills.