In the lore there aren't a large number of warlocks out in the open practicing their magic in the Alliance and Horde. Back in the day you would always see random NPCs or groups of warlocks in caves, basements, and such places congregating. Or why some warlock trainers would be in obscure locations; sinister looking locations.
I think because pandaren (in the lore) spent the most part of the game's history shut off from the world - surrounded only by the rest of their
own race - and have had centuries of time to practice their culture - will never allow them to become warlocks.
Unless, of course, something tragic happens to their society. Something like...
...invading forces waging war upon their homeland.
EDIT: Back to the topic... this could be a good way to bring "Dark Apotheosis" back into the fold. Somehow re-tool it.
Or maybe it's too close to Metamorphosis as a whole. Oh well.