I really want to believe the reason is the Humanoid Nerubian. They have a lot of work done on them so far, and they wouldn't have the dev time to do them and prep them for playability if they didn't add them into TWW before, say, TLT.
I would have loved Qiraji but they simply aren't as popular as Nerubians, so I think they just had to choose one playable Aqir race to focus on.