I think a lot of gamers just grew up. Life gets in the way for a lot of folks. Starting a family, for instance, is a big diversion.
For others, the phrase, "The more things change, the more they stay the same," just isn't literal enough. Can't handle the evolution of a series they used to devote a big chunk of their life to.
Some get bored. We've waited an average of 1.7 years between Halo games, and because not every player is h-core or can have 1337 skills (so no source of great satisfaction from playing), they move on to something else sooner than a hardcore MLG-worthy player might. Some of those don't come back.
And others… Well, this could be wrong, but from what I've seen and heard, between CoD and Halo, CoD is the more Camping-friendly of the two, thus attracting many of the unskilled and lazy players who don't want to really work for their kills.
"But that's just my opinion. Don't go spreading it around or anything."
Points to whomever knows where the quote is from.