I think cycling's popularity is a question of accessibility and the other things everyone already talked about.
To play basketball, you need a ball, hoop, and maybe an hour to play with some friends.
To cycle, you need what seems to be an overpriced mechanism, special clothes, and, if you want to go significant distance, a sizeable chunk of time. Throw in that biking through urban areas isn't much fun and that in most of the country motorists believe bicyclists belong on the sidewalk, and it's just harder to get people started on it.