You do know that there were lines outside of stores for midnight releases don\'t you? Biggest pre-seller in history, sold 2.38 million copies in its first day making 125 million in sales...FIRST DAY! That figure is for North America only, the game was released world wide in the same week.
Also, if a third of Xbox owners bought Halo, which they did, then it isn\'t hard to believe that a majority of them came back for the second game. Halo 2 is also a system seller type game so it probably brought in some new Xbox owners as well.
I just don\'t see how that\'s hard to grasp considering the original was such a popular game...