This has been discussed countless times. Numerous posters keep bringing up the Pandaren example and keep getting the same response.
Neutral Pandaren were a mistake and proved neutrality does not work, it comes at too great a cost to the identity of the faction and inhibits the story of the neutral race.
Blizzard hasn't added a neutral race in six years, one of the lead devs at the time said the concept had problems and allied races allow blizzard to introduce new faction specific races on the cheap.