Easy, it was an experiment they decided to try to see if it worked. Here's the rub though, they are pretty clear that it didn't work. Oh sure, Pandaren themselves are a fine race, but the cost to faction diversity by making them neutral was simply too great. And Pandaren had everything going for them, a race both sides wanted but neither had meaning they would join both sides at the same time, a story emphasising balance and their neutrality. Still wasn't worth it. They've not a done neutral race since. They likely never will again as the real benefits of making a neutral race, only a single pair of levelling zones and just two models to make rather than four zones and four models, is obviated by Allied races allowing model reuse and skipping the first levelling zones entirely.
Pandaren neutrality is not a precedent. It was a mistake. And the thing about mistakes is that once you make it, you can avoid making that same mistake a second time.