[ameliaquining]

They were; there's a whole complicated set of namespace reservations for government entities under .us. https://en.wikipedia.org/wiki/.us#Locality_namespace

However, state governments don't have to use this if they don't want to, and they're allowed to have .gov domains. Different states made whatever decisions for whatever idiosyncratic reasons. I strongly suspect that .gov has better name recognition as "this is definitely the government" among the American public, so that might be a reason why state and local government entities might choose to use it.

[selectodude]

Until recently, non-federal entities weren't allowed to use .gov. So states and municipalities had to actively switch (and advertise that switch).