I'm not convinced that the time difference between a 9 hole round and 18 makes that much difference.
Figure in 30 minutes to get to the course, 30 minutes warming up and getting ready to tee off, and 30 minutes getting home from the course. You have 90 minutes invested without the actual golf. The 120 minute difference between 9 and 18 isn't that significant.
In many cases, the time to and from the course may be even more that 30 minutes each way.
That said, I can't say that I can identify the reasons behind decline in the game's popularity. Bringing along new or infrequent players, I have seen their difficulty learning the process, rules, rituals, etiquette, etc. that can make them uncomfortable as newbies. I've seen struggling new players get chewed out by a marshal & vow to never return.