This continues to be a topic of speculation, with no specific reasons for the decline. This is similar to the Movie drive-in theater decline. I realize this is a different topic, but they are related, in that bowling and drive-in theaters where family institutions 2 or more decades ago. There are still drive-in theaters, but the industry is almost non-existent.
Back to bowling...
Whenever my coworkers say, 'lets go bowling,' everyone is up for the idea, because it's a one time deal
, and a one time expense. If I ask the same coworkers, if they want to join a league (winter or summer), they say no because they do not want the commitment that requires them to bowl 1 day a week, and pay 1 day a week, even if they don't bowl one week.
As you're pointed out, one time bowling events, like open play, parties, cosmic bowl are popular for just that reason. They are one time events, that don't require the commitment that drives
the recreational bowler away from league play.
When I go to the bar, to enjoy some adult beverages and play darts, even though there are local dart leagues, doesn't mean I want to join a dart leagues, simply because I am there to enjoy a once in a while moment for recreation.
Even AMF is changing how bowling is preceived by converting their alleys (where appropriate) to their 300 bowling centers, which focus on the one time recreational bowler. It's an interesting marketing idea, which understands that times have changed and bowling centers can thrive by focusing on the recreational bowling. Is league bowling a thing of the past, like drive-in theaters? Yes and no. There will always be league bowlers.