Halo's longevity is the multiplayer. The numbers 4 months after launch are the absolute worst any mainline Halo has ever had. No one besides a small portion of players were still playing the game after launch.
Probably a bit off topic, but what was changed in the overall multiplayer to have the longterm playerbase take such a hit like that anyway? Never followed the game that much, but I'm guessing they changed something for it to drop like that.