Let me tell you, I study marketing and I think they just messed up their release date. I mean, with the Halo 2 X-Box release, you know you're in trouble. If you look at it in a marketing way, they should release all the versions of the game half December (i.e. before christmas), and they should advertise it pretty well. Unfortunately, that isn't possible, because the PC version will not be finished.
From a marketing point of view, it's just bad timing/luck.