I always explained this difference between floor numbers in my country and the US by language: in my language the word used for upper floors only means upper floors, so the 1st floor has to be above the ground floor; while in English they're all floors, so ground floor is the first floor.

But I didn't know the British use the same system as my country (and most of Europe afaik). They could've just adopted the same system, despite language, for consistency.