Aside from the fact we'd be abandoning years of hard, dedicated work, WarCraft III also has a lot of resources (models, textures, sounds) that Wc3:WoW can use already and not have to force us to create every single model or texture on our own. For example, a lot of units and icons are two important things that are WoW-like and do not require the Wc3:WoW development team to create on their own. So by creating World of Warcraft in WarCraft III, we're saving ourselves a lot more time.
Yes, StarCraft II looks wonderful, but our team is too small and is not prepared to suddenly switch to another game. The only way we'd agree to create World of Warcraft in StarCraft II is if we're able to export all the models, textures, icons, and sounds that we need and use them in StarCraft II. But keep in mind that StarCraft II will not be supporting .mdx model files nor .blp texture files.