Overseer got an answer, resources could be used for other things, and I'm not sure about the beauty of 3D. In FPS, sure, but in RTS...
I'm no ttalking about resources from my computer, I'm talking about developers resources. It's more expensive a 3D game than a 2D one.
Civ 4 is an example, the 3D are horrible in my opinion, and useless. I'd prefer a 2D game with detailed graphics that a "candy" 3D engine. I repeat, I like 3D when it affects to gameplay, but not for stetics.
On the other hand, you have Myth series, or Total War. And the upcoming tactic game theatre of War. Those are good uses of 3D engine, but Warcaft III?. Waste of time and resources.
I don't like games that the only thing which changes is the 3D engine. If I'm tired of playing Starcraft, I would be tired of playing Starcraft 3D if the gameplay is the same.
Regards