Fewer people of the right age exist, for starters. The Baby Boomers are aging out of the equation slowly. Then younger people grew up in the "Age of Fear" or the "Kids Photos on Milk Cartons" era where parents never let their children leave the yard or house. Same with skating rinks and Rollerblades. Young people have so many indoor choices, plus team sports that parents take so much interest in forcing kids into soccer leagues and others. No one grows up skating anymore. Fewer grow up cycling anywhere but on the driveway if they are lucky. Sure, almost every kid gets a bike but they live in a cul-de-sac and can't get outside of that sterile environment without crossing a giant highway, then cycling on a giant highway.
So...smaller population to draw newbies from, a more frightened population within that smaller number, urban sprawl of super highways at every turn, every college grad in debt, and as always...our car culture simmering below the surface likely contribute. Changing climate may be a factor as well. More extremes of heat and cold in many locations must take a toll.
Baby Boomers are like the "pig in the snake" bulging along the length of the snake until it leaves the other end. They are much closer to the tail than the head of the snake now. I believe this bulge of sheer numbers moving on is a huge contributing factor.
