That's what I don't understand, how did the term "Caucasian" come to mean "white" in English? It would be much easier to call everyone white and leave "caucasian" for Chechens and Georgians and all those other ethnicities in the Caucaus.
Because the physical appearance of the vast amounts of caucasians are white or light skinned.
Over the years common use it became to mean "white".
And years being since the late 1700's.