this is my completely uninformed drunken gut sense response. Probably because something happened during development and they decided it wasn't needed. Honestly the appeal of a lot of these games is the multiplayer component. I used to really like campaign games for RTS's as opposed to multi because frankly I hate sucking when playing against people who play these all day everyday (no disrespect to any of you who do... lord knows many of the rest of us are envious). But as of CnC3 I decided I don't really like the scenario missions. They are all about using the next new unit on the "unit-structure food chain" in a rather contrived scenario. Moreover, I would say that the real value in an RTS is in it's multi-player or single game/skirmish mode because there is more replay value in it. I personally have always liked entering a game on even footing with the AI rather than trying to assault a base where the AI has already dug in and sends sporadic raids at me. I admit however, the first thing I did when loading the game was look for the campain mode-lol
happy trails