For cellular networks the general tradeoff is speed vs range - the higher speed protocols have much shorter range, so you need a more dense network of cell towers. Noone is going to splatter the whole countryside with 4G LTE high-speed coverage, this is going to cover the urban areas only. It's simply not practical to do it for low density areas, you'd have to put up a whole cell tower to cover just a couple people and they aren't going to pay for that; you'd use frequencies and protocols that get longer range but don't get as much speed.

It's worth noting that 5G explicitly combines very different techniques with very different properties (e.g. high-bandwidth short range systems and low power slow networks for things like smart meters), and the 5G systems that are competitive with fibre are essentially designed for dense metropolitan areas, USA won't cover all the suburbs with high-speed 5G.