I think it is a whole mess of things that lead to the decline in league membership and whatnot.
First and foremost, people nowadays at least around my area are committment phobes. Case and point increase in dioverses and couples that stay together but aren't technically married.
Anyways back to bowling. The whole one time, one expense thing rings true from my viewpoint. And this purely based on the fact that in the two years that I've been competatively bowling, I've had three people on my teams quit on me. One was for monetary reasons, and he and I (I was the captain at the time) talked about it, found a replacement and he was on his marry way. The other two nothing, not a reason at all. They just up and quit.
A lot of people view bowling as recreation, like it was mentioned and lets face it. The equipment and training to play our sport isn't exactly cheap so I think that repels a lot of people as well. Plus it takes a bit longer to be an accomplished bowler (I would say 190+). It's not like playing defensive line (no offense to D-linemen, they have gotten me plenty of sacks in my time as a football player so I have great deal
of respect for them), however Defensive linemen are stereotyped as being stupid (and occasionally it's true) so they're told to go here and hit this guy. The same can't be said about bowling, to be decent, you usually can't stand in one spot and throw the ball at one spot and get high scores. Sometimes that does happen but eventually you'll have to make adjustments. And most people don't want to take the time to learn all the different things you can do to be better. So going out, chucking a house ball for a few hours with friends and having a good time seems much more appealing than spending 200-300 dollars for equipment, lessons, and time to practice.