This stuff wasn't brought in because they simply did not have time to do it. They spent all their time developing the campaign and apparently diddling around with a "warfare" multiplayer with 32 players that amounted to like 1 piece of paper.

The reasons this stuff was cut from Halo 2 is because of time constraints, the game being rushed, and because Halo 2's development was all over the place. The new stuff is for the directors cut section of the game, so your precious Glitchfest 2 is fine.