Given this is a limited time event I totally get not doing CM sets. One it has to be tuned and tested differently than MoP. Secondly, if they make it like the original where you had to unlock per class, completionists would go nuts.
Although I agree the lack of the legendary quest is strange. My only thought is maybe they are going to add a new one, that's maybe tied to Xal'atath instead of the original?