Exactly.
Running requires...uh...let's see...a pair of WallyWorld shoes and an iPotato to post 5K finish line selfies to FaceTime #i'msoawesome! Not so much skill required beyond putting one foot in front of the other.
Cycling...not even close. As easy as it is to simply ride a bicycle, very few have the skill sets needed to get one across a finish line only 20 miles away. Add in the cost and complexity (to most Americans a bicycle is more complicated than the Mars Lander) of a bike, the maintenance requirements, the learning curve, the conditioning, the weird clothing and...well, hey! Did you guys check out my FaceTime pictures from the big 5K last weekend?
Americans also regard the bicycle as a child's toy and not one citizen in 100 could discern a big box store '10-speed' from a Pinarello Dogma/Campy Super Record EPS. Running, thanks to folks like Mr. Fixx and the millions of 5K warriors that shuffle, jog, race walk and even manage to break into a nice stride, is considered a real sport.
Running is so much easier to do in almost every aspect you can compare to cycling. Until we start talking about serious running...
It also causes so many more activity-related injuries, too...provided you can keep a bicycle rubber side down and avoid contact with 2-ton vehicles. Cycling is much easier on the body and is often the last refuge of runners that repeatedly tore themselves up, rehabilitated...and then went out and injured themselves, again.
I really wish I could run. It's an awesome sport IMO. Like swimming, it's a pure sport.
One other factor to consider is the cyclical nature of the popularity of cycling.
During times of hippie dippy green 'revolutions', high gas prices, Greggy or Lance winning that big bike race in France cycling events are more numerous and better attended. Cycling has been in one of its slumps ever since Lance got busted doing what everybody else did and still does. Easy come...easy go.
And one last point...
Cycling in America is DAMNED DANGEROUS! Despite the bikie loons in the "Is Road Cycling Dangerous?" thread, most of America...dumb as we are...have figured that much out.
And thus, upon becoming responsible adults, they avoid cycling like the dangerous plague that it is and buy large SUV's and Bro Dozer trucks to join the fray and perpetuate the dangers of the Great American Road in a game of never ending up-armoring.
Oh...did I mention cell phones? Yeah, all responsible adults stick an iPotato in their ear 24/7/365. This device not only helps increase the dangers of the Great American Road, it simultaneously blocks out any thought of every cycling past the age of 16 and kills brain cells even if it doesn't kill drivers and cyclists.