I will guess. I might be wrong.

The West I will define as Western Europe, extended to it's close allies.

The United States broke away from Britian but re-established ties during the World Wars.  So the US and Western Europe became close again.  Also, the US subsumed Japan and made it a vassal.  So Japan is part of the West

South America came from Portugual and Spain, but didn't get involved in the World Wars (maybe?) and so it remains independent from the military ties that bind the rest.  So the West has come to be a name for the allies of WW2.