Because they were holding the line halfway down Darkshore and Stormwind was on its way until Saurfang suddenly broke through and pushed into Lor'danel, at which point they began evacuating. It'd be like evacuating Stormwind because the Horde showed up in Darkshire.
There are innkeepers and vendors and stuff, but it's also the city where the Blight was first being developed and tested against captured living people.