[dragonwriter]

> Except "flyover" states are not just rural areas

Not just, no. But, looking at population density by states, you've got roughly:

(1) the coastal states way at the top (except Alaska, Oregon, and Maine), (2) non-coastal Mississippi River states, Oklahoma, Colorado, and Arizona, and Vermont in the middle (3) Everything else.

They are very different environments for things like passenger transport economics.

> There are tons of big cities in non-coastal areas of the US.

Define “big city”? There are three (out of 24 in the US) metropolitan areas with a population over 2.5 million where the principal city is located in a state without ocean, Gulf of Mexico, or Great Lakes coast; 0 out of 9 of your cutoff is 5 million.