Only because Halo fanboys will play this game endlessly. Halo 2 has such a huge fanbase for a lukewarm game. Halo:CE is and will remain the greatest Halo title of all. While Halo 3 may be graphically impressive, it will take a lot to overtake the original in gameplay value.
Don't get me wrong. I'll buy the game and play it alot myself. I just hope it lives up to the hype it's getting now and the hype it will no doubt get closer to its launch.