I was thinking about this, my humble opinion, Halo 1 / 2 / 3 were streets ahead of any game, so dominated the online market for gamers, by the time Reach came out, the competition had caught up, people moved on, people found other communities to play in, Halo basically had competition, people stopped wanting to run around the galaxy as a spartan, and wanted to run around abandoned buildings in Modern Warfare, kill zombies in L4D, save Sera in Gears, run around ancient Italy assassinating people in Assassins Creed or even tear up the streets in Need for Speed
Simply put nothing has changed in the Halo Universe its just all the other games caught up and people realised they could have fun online playing other games, other than Halo.