Because MS wanted to cram in all the DLC before the next generation launches, people no longer have much of a reason to stick it out for the game. Besides, y'know, actually playing it and enjoying it. Weren't there Halo 2 map packs launching well into... holy crap, 2007?! Wow. That [i]is[/i] late, I had no idea.
inb4its because the game sucks
inb4opcantinb4
inb4inb4opcantinb4 maybe.
I should point out that H4 was my personal GotY 2012, but it didn't have much competition. Trials Evolution was the other frontrunner.
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The DLC has nothing to do with why the game's population has dropped. It has to do with the fact that the game's multiplayer is shitty. Compared to Halo 3 which maintained a solid population for it's entire life, halo 4 is dropping off in pop. because it sucks ass. End of story, I don't give a shit what you say, but I jumped ship in the first month because it was boring.