I've been wondering this for a while now. Caucasians seem to refer to the caucuses who's origin can be traced to Slavic peoples. They may have a relation to indo-european ancestry through their language but even that is not completely known to be true. So why did this term expand to cover all of western Europe regardless of ancestry or origin?
Why do we call ethnic Europeans Caucasians? written by u/NutBananaComputer may interest you
I would like to correct wrong assumptions in OP post; Caucasus origin can't be traced to Slavic peoples, the region is ethnically very diverse, but Slavs (namely Russians) only entered in the region in 19th century through conquest.
Slavic languages are without any doubt known to be indo-european languages, I just wonder where you get the idea that it's not completely known to be true.
Now, here's a "funny" part regarding US usage of the word "Caucasian". There's a slur in Russian language for Caucasians, which is considered pretty offensive, so I'm not going to name it here, but it translates as "Blacks".