I wouldn't put it in those terms. As others have already indicated, the Xbox 360 population is considerably more diverse now and there is a drastically wider selection of competition.

Strictly speaking, CoD Black Ops 2 outsold Halo 4, despite the typical RRP being higher.
Battlefield 3 also gets quite regular massive updates (Seriously, multiplayer 'updates' alone for BF3 are taking up 8gb of my HDD!) which is taking it's chunk of the less casual CoD players.

Another possible way to look at Halo 4 is that despite the quite vocal complains on this particular forum, the player investment system is actually very brief.
Between fairly play and double XP codes, people were reaching SR130 within the first month.

That's just not possible in the likes of Gears of War or CoD. The considerably longer and more grindy player investment systems actually seem to succeed in keeping people playing for longer.
As much as some of our members deride it, it's no wonder Halo is slowly starting to follow suit (albeit requiring considerably less 'investment' overall) since the figures seem to show that the majority are quite happy to go through it.

After all, Reach had a pretty absurdly long investment system with some rather meaty rewards that seemed tailored to appeal to the 'underground' movement.
Choosing to have a skull visible behind your visor is the kind of thing a fair chunk of people will have a really good go at trying to earn.