All the expensive, but readily accessible leisure sports have seen large increases over the past three decades. Two of the major factors in this trend are changes in income distribution and the greatly decreased cost of travel.
The former means the traditional base from which divers are drawn (what in the U.S. we call upper middle class) have much more disposable income than before, so are more likely to try out an activity even if it costs several hundred dollars. Combining this with the much lower costs of travel means that the Carribean or Hawaiian vacation, which used to be considered a once in a lifetime trip for most Americans, can now be an annual dive trip.
Of course the industry has responded by making gear more accessible and dumbing down basic training. The leisured classes simply will not put up with many too many barriers, they can always play golf, go fishing or hunting or snowmobiling or ....
The counter example would be flying. The number of active pilots has steadily decreased in this country since the WWII generation finished their government funded training (either directly or through the GI Bill). Why? Since flying is tightly regulated by the government, which puts a much greater emphasis on safety than increasing participation, training has actually gotten harder over time. The leisured class is simply not willing to put up with a couple hundred hours of study (including, gasp, math) and training. This, by the way, is why diving is so anxious to keep the government from regulating it.
Aviation is the counter example for gear as well. For a variety of factors (OK two factors, liability fears and the government approval process), until a few years ago, private aircraft evolution had virtually stopped. While dive gear was getting safer, more comfortable and more colorful, factory aircraft were basically stuck in the pre-electronic era. It got so bad that all real innovation was coming from people building experimental planes in their garages. Finally decades after the garage tinkerers came up then, things like composite construction and whole plane parachutes are making their way to factory built aircraft. This has led to a mini-boom in sales, but of course without more pilots the effects will be limited.
It's interesting that the aviation industry has finally convinced the government to meet it halfway in an attempt to bring new people into the sport. This has resulted in a new Sport Pilot license which takes the rec diving approach of maintaining sufficient safety despite providing minimal training by simply limiting allowed activities to those least likely to cause problems. Just like rec diving is limited to NDL depths and no overhead environments, Sport Pilots are limited to slow, small and simple planes and flying during daylight hours in good weather. There is also a new category of Sport Pilot planes. These get a much simpler approval process, which should allow for greater and faster innovation than under the existing regulations.
Alex
