One problem here is that I think there are two different definitions of "rural" and two very different reasons that people would use transit. "Rural" is a hard word to define, and lately it has been more of a cultural badge than a geographical description.
I've lived in places like Montana, and sometimes I would talk to people from Indiana, and they would say something like "I lived in a tiny town in the middle of nowhere---only 10,000 people, an hour outside of Indianapolis". In most of the Great Basin, Rocky Mountains or Great Plains, a county with 30-50,000 people is going to be an "urban" county. For example, it is 1100 miles by car between Fargo and Spokane, and in that distance, the largest county someone passes through is Yellowstone, home of Billings, population 160,000 people. That would be, for example, the 19th largest county in Ohio. It is really hard to explain what the Great Basin is like until you have been there. Between Lakeview, OR and Pendleton, OR, someone can drive 335 miles and pass through one town large enough to have a supermarket. (Although
@oregon pioneer might correct me on that).
Compared to that, areas like Ohio, Indiana, Illinois and the like don't have extremely isolated rural areas. They have rural areas that still have high enough populations, and high enough population densities, and that are close enough to major urban areas that they can support travel. Maybe not for daily commuting, but for things like weekly shopping trips or medical/etc appointments.
So you have two very different rationales for rural transportation, depending on what type of rural it is.
In areas like the Montana Highline on the Empire Builder, or the California Zephyr, along with tourism, people there often don't have any other means of transit.
In areas like downstate service in Illinois (or corridor service in Michigan, Wisconsin or Missouri), those areas have high enough population densities that for practical purposes, they can be considered almost exurban. They are all at least within an hour of a small metro, so they can sustain commuting.
(Incidentally, if anyone is interested, the USDA actually has a definition of "Frontier and Remote", which is any area that is more than an hour from a metropolitan area of at least 50,000 people. Areas like that are more than 50% of the US' land area, but account for about 8% of the population. Significantly, none or almost none of such "rural" states as Ohio,Indiana, or North Carolina have such territory.)