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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For how long after a game comes out that I will regularly play usually depends on a number of factors like playability and how fun multiplayer is. However, when DLC is involved, the amount of time from the release of the game until the last pack of DLC comes out, divided by .75, is usually how long it will take to play through all the DLC before becoming bored. In the case of Halo 4, that distance isn't even one year, so I think all but the most dedicated Halo 4 players will be gone by January of 2014 :(