If you beat a level in a typical RTS, you get more story, generally a cutscene or two, some new units perhaps, etc. You're progressing through something that keeps you interested. In civ, see everything the game has to offer within a 2-3 hour stretch. You don't have any story, cutscenes, or new stuff to look forward to, just yet another pop-up of George Washington asking if you'd like to trade your fusion technology for everything he knows about pottery.