How were they built like "unplayable races"? There's nothing disqualifying them from being playable. And there's plenty of reasoning why they would be playable.
We already know the story premise of Boralus and how it ends, the Alliance goes there to convince Katherine Proudmoore into joining the Alliance. Taelia explains that while lady Ashvane sits at her side, she won't join. Later we learn that lady Ashvane is a villain and wants to usurp Katherine with the help of the pirates in Freehold and is working with Queen Azshara. Ashvane betrays Katherine, who then starts to consider joining the Alliance. Just as the Horde recruits the Zandalari for their great naval fleet, the Alliance approaches Kultirans for the same reason.
The whole reason the Alliance goes to Kultiras is to recruit the kultirans.