I think there are some elements at play that will keep cycling from ever being as popular in the US as it is in Europe. One of the main things is the average person is VERY hateful towards cyclists and see us as a nuisance when we are out on a ride. Unless there is a cycling lane in your town, I'm sure most of us have had the evil looks, drive-by cuss out, or even the occasional bottle thrown at us. Anytime there is an article in my local paper about a cyclist being hit by a car, the majority of the comments in the comment section are along the lines of "he deserved it" or "too bad I wasn't the one who ran him over" or "well he should have stayed his a$$ off the road." It is really sickening. Then again, in many cases, cyclists tend to have a certain "I own the road" mentality that drives commuters crazy. So, as long as there is a widespread endemic of hatred toward cyclists, I think the sport will not catch on.
Next, Americans are by far a rather lazy sort of people who tend to enjoy watching sports on the TV rather than actually participating in them. And, as stated in previous posts, cycling is overall a slow paced sport. I think it's safe to say that the majority of people who watch cycling are cyclists. This is the complete opposite of other sports. There are plenty of football (American football) fans who watch games but rarely, if ever, play it. The same can be said for basketball, baseball, hockey, etc.
Lastly, cycling is an expensive sport/hobby. The bikes are expensive. Tires, saddles, pedals are expensive. High quality clothing, shoes, helmets are expensive. If you don't know how to maintain your bike, taking it to a shop for maintenance or repair is expensive. It's not like going to ****'s sporting goods and plopping down $20 for a basketball and going to a court for a pickup game. The price alone is a turn-off for many people. I mean seriously, there are road bikes that cost more than a motorcycle or even a good used car! Madness, I tell you!