Because firefight uses ODST models not Spartans and the menus are different, which wouldn't be compatible with the Halo 3 menus.
So ODST features is on disk 1, Halo 3 Multiplayer, Forge and Theatre mode is on disk 2.
Destroying the world is a lot like eating yoghurt, you use a spoon.
JMO Lord of Spoons
GRAVEMIND
Want to contact me for any reason? My Gamertag is: JMO 137