To be fair, I'm not -still- stuck on it. I haven't had the chance to play on XBox Live in almost four years, and when I did, I picked up Halo 4. And it sucked, badly. Along with adopting everything from COD that I didn't like, they also abandoned everything that made Halo's multi-player unique and managed to screw up many, many custom games in the process.
The only reason I'm playing Halo 3 is because it is the Halo that I remember playing. While I can't say that I disliked Reach, it wasn't as good as Halo 3. And my copy has been delayed several times in the mail.
Not only that, but how many people still play Counter Strike? The only reason people aren't playing Halo 2 right now is because of Halo: Reach and Halo 4. Otherwise, the fan-base would be divided, and not too unevenly, between Halo 2 and Halo 3.