The stone (14 pounds, about 6kg) sounds like it should be a really ancient unit of measure, and it is. But it was only added to the UK's Imperial system in 1824, several decades after the U.S. had left the Empire.

Update: Although today we often call the U.S. system of measurement "Imperial," that's technically wrong. We're actually using the "U.S. Customary System," a variant of the earlier British avoirdupois system, which shares many unit names with Imperial...but not the stone. (Source: Weights and measures standards of the United States, Lewis Van Hagen Judson, 1963)

I don't know how far back in time you want to go but asking about a current situation might fall afoul of the subreddit rules.

why is the stone used in some European countries when they are based on the metric system.

Where's that? I'm not aware of the modern "stone" referring to 14 pounds avoirdupois being in common use anywhere outside of UK and Ireland