Clearly, one of the biggest factors in flying is the cost.

Some, maybe. When I began my private ticket in 1973 I paid $24
per hour , dual, whcih was about a day's pay for me at that time, at 20
years old. Now, we charge about $150/hr dual, same sort of airplane,
which is a bit more than what the average 20-year-old around here
collects in a day. The difference, I suppose, is the lawyers' share.
Of course, with the oil/gas boom we're having in Alberta, there
are kids making $25 and $35 an hour, but they're buying $60,000 trucks
and big houses, not flying lessons.
Much of the big burst in flying came when the kids who were too
young to fly in WWII got old/rich enough to take lessons. They'd
watched the newsreels of the fighters and got the bug. Flying is now
more than 100 years old and the novelty has worn off for the younger
generations. Further, many of the PPLs I knew in the '70s get their
tickets, then flew another 10 or 20 hours until the family realized
that there were more urgent things to do with that money. Flying around
for an hour on a weekend gets a little stale for many unless there's a
more significant point to it.
Some of the reluctance of the public to start flying is their
perception of us as pilots. We can be arrogant and snotty, as if we're
so superior, and it turns them off. Most men would take lessons if they
had the chance; I get a lot of new aquaintances telling me they'd
always wanted to get their Private License. They seem to think that
they're somehow incomplete without having mastered it; perhaps some
discreet advertising along those lines might work. Plenty of other
sports use it.

Dan