Mostly because busy roads with high speed limits - on which bikes obviously don't belong if there isn't a separate bike path - have a way of being, you know, between two or more cities. And in rural areas, particularly in the US, distances tend to be too great for bikes to be very practical.

Of course there will be particular roads in cities where it's dangerous for bikes to ride, but on the whole, there are obviously going to be more cyclists in the cities than in rural areas. And since cars have to be more careful and drive slower in cities than in rural areas anyhow, that works out nicely, doesn't it just?