-
Because disk and disc aren't even homonyms they are the same word spelled differently. People use the k one for old style floppy disks and the c one for optical disks but it is convention not definition, I think. That is why I mention canon (there is probably a better non-book-word word for that but I couldn't think of it*).
English, The only rule is: There are no rules.
* although clearly I just did. The word is "convention".
compact disc, floppy disk. At least in canonical terms. @miro_o