And then it dawned on me: the Postal Service has a mandate to deliver mail to every person in the United States. That means a Post Office and a Postmaster and probably a Mail Carrier in every small town. Other delivery services can focus on profitable markets and let small-town customers make the drive to the nearest big town if they want to use their service.
Pure speculation, of course -- my specialty. So I was extremely gratified to read this analysis which draws the same conclusion (and points to an outline of some other significant factors).
Update: Here's an Economist article that makes many of the same points.