I think that has to do with the Roman occupation a few thousand years ago. ;)
It appears the practice started back when we were still part of Great Brittan, but never became formalized until 1792 with the advent of the original Pennsylvania Turnpike.
Their is an interesting article about it at
https://www.fhwa.dot.gov/infrastructure/right.cfm
FWIW, I think most Yank cyclists follow British tradition because they seem to prefer riding against traffic while pedestrians walk with the flow: both in violation of rules of the road.
I know, you expected something pithy and humourous, but anything I said would end up insulting the country, the people, those untold number of family members, the Queen and her little dogs too.
And I would never insult a Corgi