I played Halo and its sequel all the way through, as for three I never did pick it up but I was aware from asking a friend of the fate of master chief. But even from playing just halo one and two I know that even with master chief no longer being a part of the series that there is still plenty of room to advance in the series it was a all out war in Halo one between the human and the covenant and if I'm remembering correctly the flood was also introduced already in Halo one so just in the first game there are three factions going at it head on. Just because one soldier dies no matter how legendary to series he may have been the game has so much more story to tell.
And money can't forget to squeeze all the juice out that you can. I would like to put Halo above that but come on lets be realistic.
It would help if I knew what game it was that you were talking about. Mainly because some games require two discs due to the vast amount of content they may have. Or if its not a game with a great amount of detail could of just been a simple mistake.
Explore the world. It's massive with tons of side quests every where you could look. The games main story line ends fast but it still has all the guilds to join the huge world to explore and all the side quests.