The rural hospitals typically serve a population that otherwise would be several hours travel from the nearest "real" hospital, and typically have a catchment area of 1-2 hours travel. Many of these hospitals are located in towns with populations of a few thousand people. Even when heavily subsidized, the economics of maintaining these hospitals is very poor, especially as populations decline. Maintaining a fleet of doctors and facilities that are increasingly rarely used is expensive. And even with the current distribution of rural hospitals, it is not uncommon for people to live several hours from the nearest hospital.
It is essentially a math problem. To ensure that virtually everyone lives within N hours of a hospital, you would need to build an enormous number of hospitals that each service a population of a few thousand people unless N is quite large. This has an extremely high cost per capita, most of which is wasted because they are often very underutilized, which does nothing to help reduce healthcare costs. And you can't force these people to move to more urbanized areas of the country either. The other challenge is finding enough doctors willing to work in a tiny remote town. Few people want to live in these areas and so many doctors that do work in these hospitals are effectively required to commute from a city that is hours away -- it is not a desirable assignment for most.