Because it was a bad game.
-There were too many off-spawn abilities that absolutely cripples map and game flow, such as Armor Lock, Camo, and Jetpack.
-There was only a ranking system in a single playlist; players would be matched up randomly, regardless of skill or connection.
-A high degree of random elements were added in. Grenades would do random damage and have inconsistent fuse times, especially when insta-sploding Plasmas.
-Many of the weapons were too weak for their own good, and went unused. The DMR was the king, despite it being a bad weapon itself.
-Reach required teamshot, as 1v1 skill was very low.
-Movement acceleration and aim acceleration made strafing near, and aiming against an ascending Jetpacker respectively impossible.
-Bloom.
-Vehicles controlled terribly, with small arms weapons able to flip Warthogs around like they are made of cardboard.
-Overpowered Banshee.
-The story was bad, with flat uninteresting characters. The only one that Bungie decided to add any character to died halfway through.
-The credits system killed objective games. Since there was no ranking based on skill, it was advantageous to just go for kills the entire time.
-Firefight was made too easy, with many casual gametypes added later. Firefight was abused to gain a high amount of credits at a time.
-The game shipped with bad maps.
-MLG was a social playlist.
-Team restrictions were all but nonexistent. Full teams of Inheritors could match someone who literally got the game just then.
Need I go on?