In the 14,000 years since Dragons came into the world a number of things have happened. It's important to remember that for a classical triple Bond (two Elves/humans, one Dragon) there has to be someone available and suitable (ie, a Lightborn or someone capable of being one) for the dragon to choose. When the last Elven Dragonmage died, all the bonds were Dragon/Lightborn. Lightborn spells cannot be written down or passed orally. All it would take would be a few missed connections for the Triple Bond itself, and then the knowledge of it, to disappear.

(And BTW, I think that Kellen is a wonderful name for a baby.)