A campain, is not just maps linked,is a tale, that if told right with good cutscenes can make a game a very good thing.
Yeah, but how often are you going to play the campaign? Once. Using up developer time for a feature that most won't spend much time playing is a waste.
Let's talk about those 'icons of RTS' and their campaigns. How often did you finish the Starcraft campaign as opposed to multiplayer? Age of Empires? Company of Heroes?
On the other hand, look at the 'icons of TBS'. Civilization. GalCiv. Massive Assault. Master of Orion. Master of Magic.
They have no campaigns, instead they are purely sandbox / multiplayer. (Yes, GC and MA have campaigns, but hardly anyone plays them, hence it got dropped for MA Network.)
There are a few games that use a different approach (the UFO / XCom series), where it's dynamic campaign. Steel Panthers as well, although that game shines more in PBEM sandbox battles.
Maybe I'm not the typical RTS player but I play (and have played) the campaigns of "classics" quite often. Honnestly, I must have finished each of Starcraft's campaigns a dozen times at least, same thing for warcraft 3 and frozen throne, or the C&C series, etc.
I usually don't really play multiplayer games until I've mastered singleplayer mode against the hardest settings. Hell, I don't even find multiplayer that much exciting (maybe I just lack that "competition spirit")...
However, you're giving a good exemple with UFO/X-COM of the kind of "campaign" I'd like to see in Sins.