Why did this word fall out of common usage in American English? It’s beautiful and to me explains what “state” (of mind) a government should rightfully be operating within.

The English noun commonwealth dates originally from the fifteenth century. The original phrase “common wealth” or “the common weal” comes from the old meaning of “wealth” which is “well-being”. The term literally meant “common well-being”. Thus commonwealth originally meant a state or nation-state governed for the common good as opposed to an authoritarian state governed for the benefit of a given class of owners.

{See also: “a political unit founded in law by agreement of the people for the common good“}