The beauty of cycling is that it can be enjoyed on many levels and more importantly anywhere there are roads and/or trails. It can be a be an important and integral part of your day to day lifestyle. Or it can be a release from that lifestyle. Cycling is enjoyed by people from the age of 2 or 3 right up to and just before the geezer disease gets us. There are no hard and fast rules. It is not expensive to get into but offers the chance to suck huge dollars if the cyclist is so inclined. Cycling is pretty much an equal opportunity activity.
As to it's popularity. It's popular. But it has been around so long that it has woven itself into the fabric of our culture. Once entwined, it disappears.