There are so many reasons stated here that are valid and each in their own way have contributed to bowlings decline over the past 20 years. I feel speaking on my observations in my house, that the start of bowlings decline in the early 90's is a result of several factors.
1. The length of bowling season, this has always been a major complaint with bowlers. We have slowly let leagues reduce from 35 weeks to approx 32 to 33 weeks. Basically start after labor day and end at the end of April, though I know some bowlers would like to see leagues get down to about 26 to 28 weeks, but in my area that would financial suicide as we don't have a large population base to support us.
2. Changes in lifestyles (especially for women). Yes women have worked forever, but now more than ever, back in the 70's & 80's women weren't working to the large degree as they do today. Where bowling was a great nite out for stay at home mom's, a stay at home mom today is few and far between. Many women today have their babies and are right back out to the work place weeks later. My women's leagues today are virtually senior citizens leagues. Peoples work hours seem to have had an impact also. Years ago a major employer started 12 hour work days, soon other employers also adopted this and this contributed to another loss in bowlers.
3. More activities for youth, soccor, lacross, these activities didn't exist here till recently, as well as other sports and activities, both boys and girls, more activities that take youth away from trying bowling(we don't have school sponsored bowling programs, few in this state do). So there just isn't the new blood available to replace the aging bowlers who are dropping out.
4. I know this problem has existed forever, the High average bowlers, who do nothing but whine about spotting lesser bowlers pins, especially newer bowlers just getting started, instead of trying to help and encourage newer bowlers, these pin heads make the newer bowler feel unwanted and thus drop out of bowling after a short time. No proprietor likes to lose customers, but some customers are pure poison and the sooner you get them out of the building the better off everyone is. And with the explosion of ego-maniacal house hacks due to todays inflated scoring conditions and bowling dinosaurs who don't have the patience to reach out to new bowlers and teach them the etiquettes of bowling in a manner which is not condescending or nasty, no matter what their skill level is, this problem maybe more prevalent now than it was years ago.
Anyways these are simply my observations from my area and from reading comments posted from other bowlers