Why is it an American movement? Because Formal Verse always remained Okay in the UK. Probably it was Okay in the whole Commonwealth, for all I know. But in the US there was a stretch of time where it was Free Verse Or Nothing!
(Yes, there were some dinosaur-like exceptions, but they were frowned upon.)
So, why were the English different than us? I don't know.
For some reason they thought of Formal
as still being perfectly normal,
rather than treason to the cause
of flouting outmoded laws.