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.
<{POST_SNAPBACK}>
Not entirely true, in Europe games aren't advertised as much (save for specific magazines) and they sell.
Not everything is up to marketing.
There is also choice.
And yes by being the sequel to the game of the year does save a lot effort for branding.