Why is "there" a subject while "here" isn't?
I wouldn’t go along with that. Locative “there” is an adverb (some grammars call it a prep) rhyming with “dare” and meaning “in or at that place”. Dummy pronoun “there” on the other hand is pronounced unstressed with a reduced vowel and used to fill the syntactic subject position in existential clauses. So there is a difference in category, pronunciation and meaning.
Historically, dummy pronoun “there” derives from the locative “there”, but it has been bleached of its locative meaning and reanalysed as a pronoun.
The point is that the dummy pronoun “there” is without doubt the syntactic subject in an existential clause, no less than “it” is the subject in an extraposed construction. This can easily be proved:
“There” occupies the basic subject position before the verb, e.g. “There was a nurse present”.
In subject-auxiliary inversion constructions it occurs after the auxiliary, e.g. “Was there a nurse present?”
“There” occurs as subject in interrogative tags, e.g. “There was a nurse present, wasn’t there”?
Yes, the Oxford Online dictionary does indeed give existential “there” as an adverb, but it is wrong! As usual, it is just using ‘adverb’ as a classificatory dumping ground for any word that doesn’t easily fit into one of the other word categories. The examples above demonstrate without doubt that existential “there” is a pronoun. In any case, the function of subject can’t normally be realised by an adverb.
To complete the syntax, the subject of the non-existential construction becomes a displaced subject in the existential version:
[1] "Several windows were open". [2] "There were several windows open".
In [2] “several windows” is analysed as a displaced subject (an internal complement of the verb), but it does correspond semantically to the subject in the non-existential counterpart [1].
Finally, you asked why "here" could not be the subject in:
"Here was not much snow".
"Here" is not a pronoun here, but an adverb (some call it a prep) so it can't possibly be subject. The syntactic subject in this example is "not much snow", and "here" is locative predicative complement. Think of it as "Not much snow was here". As further evidence, note that inversion would not be possible, *"Was here not much snow"?
A brief look in the google on the search string
"dummy there" "Old English"
finds a few references like The Syntax of Spoken Indian English, which despite its title discusses the Old English origins of there as dummy subject. The references are to primary sources, which I haven't (and am not qualified) to evaluate.
But if the Old English þǣr, (there) had usages as both a locative and a dummy subject, but hēr (here) didn't, then it's no surprise that modern English is the same.
Of course, this serves to push the question back to Old English.
Some etymology might be useful:
there
- Indo-European pronoun stem: te, to
- proto-Germanic pronoun stem: þe, þa
- Gothic: 𐌸𐌰𐍂 (þar)
- Old English: þær, thēr
- Old Saxon: thār
- Old Frisian: thĕr (Dutch: daar)
- Old High German: dăr (German: da, dar-)
here
- proto-Germanic pronoun stem: hi
- Gothic: 𐌷𐌴𐍂 (hĕr)
- Old English: hĕr
- Old Saxon: hĕr
- Old Frisian: thĕr (Dutch: daar)
- Old High German: hĕr, hiar (German: hier)
In both of these (there, here) we have the common indo-European locative suffix -r or -re.
Compare the where the locative suffix is missing, although the Indo-European stem to is the same:
the
- Indo-European pronoun stem: so, to
- Sanskrit: sa, tad
- Gothic: 𐍃𐌰 (sa), 𐌸𐌰𐍄𐌰 (þata)
- Old English: se, ðæt
- Middle English: ther
- Old Saxon: thĕ, thie
- Old Frisian: thi
- Old High German: der (German: der)
There and the are related, here is not.