Eldest Draconic

Dragon languages do not change or drift naturally, so why are there at least two "parent" languages to the Latter Draconic spoken by all modern dragons?

According to the most reliable of the dragons, the first of all dragon languages, the Eldest Draconic, was given to them by the One directly and taken away because of a sin the dragon would not disclose. Even the memory of its sounds and the shapes of its words were wiped out, replaced by Old Draconic.

This Eldest Draconic was never reduced to written form, so there are no traces or hints of how it sounded or how it worked, but the dragons insist it existed nonetheless.