I doubt there's anything more to it beyond nostalgia. Onyxia and Ragnaros were the original final raid bosses from launch, while the Lich King was the most iconic villain we've fought to date and the big threat hanging over us after WC3. They're all great choices to use to reference the history of WoW. The only other boss I feel should have been included in that regard is Garrosh as the best villain developed throughout WoW itself, but he's represented in Remix right now.