Which, by the way, has been doing pretty brisk business during the same time period that scuba has been stagnating.
And if you want to say "but cycling is cheaper than diving" you simply need to price a set of Zipp Firecrest 404 wheels ($3,500) or swing by a popular local cycling route and count how many new Colnago C59's and Pinarello Dogma's ($6k-$10k) or even pedestrian Trek Madone's that range from $3,000 up to $10,000 go rolling by. If one of the guys (also largely guys) riding one of those bikes stops, ask them when they bought that bike, what they had before that, what they're upgrading to this year, and how many other bikes they own. (The correct number of bikes to own is N+1, where N=the number of bike's currently owned.) Take note of their $1200 power meters, $500 computers, $300 shoes, $150 jersey, $200 shorts, $200 helmet, $100 carbon fiber water bottle cages, gloves, glasses, etc, etc. Ask, by show of hands, how many of them have a $350 trainer at home, multiple sets of wheels, a $500 bike carrier for their car. And it will probably run 50/50 in terms of how many spend a few hundred bucks a year on service vs. how many have spent $500-$1,000 or so on a stand, tools, parts, lubes, etc in order to service their bikes themselves.
I've spent far more in the past few years on road cycling than I have on diving.
Sure, there's no training/certification barrier, and no significant travel barrier, but the economy hasn't hit recreational cycling in anywhere near the same way it's hit scuba diving.