City/state/US is logical, the problem is most other hierarchies confuse people. For instance k12 subdomains for schools couldn't use that nomenclature because school districts do not map cleanly to towns. And that's before you talk about fire departments, townships, libraries, park districts, and countless other governmental bodies and districts which have overlapping boundaries of their own.

.gov certainly cares a level of exclusionary access that isn't really true of .us. Only one entity, the US federal government, can decide to hand someone a .gov address. And generally there is few signals harder to fake or impersonate than one.