Interesting, did not know that. He implies (but doesn't say flat-out and it could indeed be a retcon) in the foreword to The Hobbit that it's deliberate:
In English the only correct plural of dwarf is dwarfs, and the adjective
is dwarfish. In this story dwarves and dwarvish are used, but only when
speaking of the ancient people to whom Thorin Oakenshield and his
companions belonged.
Re:The dwarfs of our vocabulary
by thaumasiotes on Tuesday August 15, 2017 @06:38PM
(#15023383)
>> In English the only correct plural of dwarf is dwarfs
etymonline points out that the form with a better-in-some-sense claim to be "correct" is dwarrows, not "dwarfs". But that term was uncommon enough that it completely died out. ( http://www.etymonline.com/index.php?term=dwarf )
As I mentioned sidethread, "dwarves" is a natural extension of what happens with other common English words ending in f, making it difficult to argue that it is somehow incorrect.
> etymonline points out that the form with a better-in-some-sense claim to be "correct" is dwarrows, not "dwarfs". But that term was uncommon enough that it completely died out.
This form appears in LOTR in "Dwarrowdelf", given as the translation of the Dwarvish name "Khazad-dûm" of Moria. https://en.wikipedia.org/wiki/Moria_(Middle-earth)
You can look at ngrams. It's much lower pre-Tolkein but "dwarves" exists:
https://books.google.com/ngrams/graph?content=dwarves&year_s...
Dwarvish is a more unusual world and it doesn't surprise me that's almost exclusively tied to Tolkien and derivatives.
ADDED from the link:
Tolkien himself admitted that “dwarves” was a misspelling. In a letter to Stanley Unwin, the publisher of The Hobbit, he wrote:
"No reviewer (that I have seen), although all have carefully used the correct dwarfs themselves, has commented on the fact (which I only became conscious of through reviews) that I use throughout the ‘incorrect’ plural dwarves. I am afraid it is just a piece of private bad grammar, rather shocking in a philologist; but I shall have to go on with it."