Because the world doesn't revolve around WoWbut why would they make a sc2 when they have WoW and BC out now?
...Though realisticly, there is a low chance that they will make a Starcraft 2. I mean, WoW is making them BUNDLES of money. Tonnes of it, more than a lot of people could imagine. But making SC2 would be a good idea, they are still making such an incredible amount with WoW, that they may not even bother with spending time on a SC2.