Basically, I did it that way because road travel just isn't very safe in most places. For merchants, sea and air travel is THE mode of travel. The rail lines are used for military transport for the most part, especially in the Sarantine Empire, where heavily armed trains travel from the silver mines to the capital. Only in Orubeth, the southern parts of the empire, and some small areas of the eastern republics are there well-maintained roads.
The atmosphere and setting is in some ways reminiscent of King's Dark Tower; most people are starting to become very insular and almost xenophobic, not traveling far from their homes as society, law, and order start to break down.