It costs more to deliver to remote rural areas than it does to metropolitan centers. The costs per delivery are higher due to the low volume and remote location.
If free enterprise takes over, it would cost a lot more to deliver an envelope to Kodiak, Alaska that it does to New York City (which is the way it should be).
Also the subway ride would be $27.49 each way and the rural areas would have less taxes. Same holds true for buses.