It's the pinnacle of the 360 lineup. When people buy a video game system, generally they buy a game with it. What are people more apt to buy when they pick up a 360... Halo 3. Thus it continues to sell.
Can really compare that with almost any system even the Wii which already has a game bundled with it. You'll notice throughout sales some games that will have pretty decent "legs" as we call it here. That has to do with either continued hype of a game while current userbase picks it up, or because this is a game that is causing systems to be sold. Mario Kart Wii, Brawl, and Wii Fit are very good examples of that on the Wii. Super Mario 64 was a great example of the case on N64.
Most people don't seem to recognize, though, that outside of Japan games with legs generally are your system sellers. Well specific ones of course. Even today games like Halo 3, Mario Kart Wii, etc are system sellers.