Bicycling isn't some magic unique activity but it does have certain attributes that separate it from many (most?) other forms of exercise.

The key to any exercise program and health is actually doing it. Bicycling, pretty uniquely is something that people actually do for multiple hours per day, for weeks, months, and years. There are other factors such as being low impact. But it's really about staying with it and cyclists do, while folks engages in other physical activities don't.