Why didn't they give us Broken Draenei

[Thage]

Opening up character customization options/adding a faction of Draenei that have been cannonocally on the alliance since BC is neither nonsensical nor a zombie dragon raid.

I still think a zombie dragon raid would be pretty awesome. Onyxia in BWD gave me the thirst for it, now it must be slaked.

On-topic: I have no doubts we'll get Broken before too long, but right now the whole point behind Allied Races is that they're easier to produce than full races, making it so Blizz can pump out more per expansion than full races. The Broken on Argus lack a female model, which would necessitate a full build even if they used female tauren as a base, as well as expansive customization for both males and females, at least until they're on the same level as Lightforged. Lightforged, meanwhile, are largely a matter of texture edits, new hair/horn options, and a permanent sigil rather than the temporary one normal draenei get with Gift of the Naaru. Lightforged were also feature-complete already in 7.3 as an NPC race, meaning no extra work had to be done to get them player-ready.

[Hawkknight97]

Broken Draenei will be added in later during the Expansion when Prophet Velen is the one to give us allied race of Broken Draenei.