Thinking about all the city riders I see on a daily basis, and having spent a little time in a shop that sells ALL kinds of bikes watching customers come and go, I'd say the number one reason they are popular is their low cost. Most of the folks you see tooling around town on single speed bikes are NOT bicycle- or cycling aficionados. As a means of getting around, they buy bikes like most people buy plane tickets: whatever is cheap, convenient, and reasonably reliable - in that order.