Because panda's weren't around before that movie.
They also probably thought it was a joke because the Pandarian race was created as an April Fools joke years ago. They've said they were never gonna be an actual race, but Blizzard has said a lot of things that ended up happening. Like being able to fly in Azeroth. This game has only got 2 more expansions left in it. It's no surprise they're trying to pull people back in with adding this race they said they'd never add.