Is it because they all came from the Caucasus? Or is there another reason? Also, If this is the wrong subreddit to ask this, then please point me to the right one.
edit: I wrote about this in much greater detail here but a tl;dr can be found below.
Blumenbach (1790) claimed that Adam and Eve were Georgian (Caucasian) and therefore white. All races stemmed from whites, and other races could be explained by denigrating factors compounding over time, like poor diet and sunlight. Therefore whites are still Caucasian. This was contrary to Meiners' theory of progenity, who believed that major races (whites, blacks, etc...) came from separate origins (1785). Meiners was actually the first to coin the word Caucasian, but it was Blumenbach who is chiefly credited with its modern use.
Johann Friedrich Blumenbach, Collectionis suae craniorum diversarum gentium illustratae decades (1790)
Christoph Meiners, Grundriß der Geschichte der Menschheit (1785).
I would suggest inquiring over at /r/askanthropology. History deals more with recorded human events and their interpretation. From that sub's about panel: "Biological anthropology is the study of human evolution and physical diversity." which exactly fits your question.
Here's a very interesting Supreme Court case in the U.S about Caucasians being white. Only free white immigrants could gain citizenship (changed in 1870 to include those of African descent) up until this point. Bhagat Singh Thind claimed that he was a high-caste Hindu, and therefore Caucasian, and therefore white. The court conceded that anthropologists considered high-caste Hindus (Aryans) Caucasian (as race was based on skull structure and not skin tone), but the language in the bill intended to use Caucasian in the popular definition meaning white. Therefore Thind could not become a citizen, and because of this decision, South Asians who already had become citizens had their citizenship revoked.
Interestingly, Thind's lawyers argued that Thind didn't want to have to marry other Indians (remember, interracial marriage is illegal) because he regarded darker Indians in the same way white Americans looked at black people.
As a follow-up, why aren't Hispanics and Jews considered white?
Maybe /r/etymology could answer this better.