LotR --- Appendix E -- Writing and Spelling

[Lalwendë]

Originally Posted by davem

I can only speculate on Tolkien’s reasons for his use of C instead of K: probably because ‘Celeborn’, as written looks better than Keleborn: ‘K’ looks too sharp & angullar. ‘Celeborn’ looks more beautiful than ‘Keleborn’ (or ‘Seleborn’). In a written narrative the shape of the word as well as the sound plays a part: ‘Celeborn’ is a more attractive collection of letter shapes than, say ‘Ugluk’.

I think it is from the use of "C" in celtic languages as a hard "C" rather than a soft "C". The most famous example is probably Celtic FC; the football team is known as "Seltic", but the culture it's named after is "Keltic".