Actually, you can sum this up in two statements.
Businesses compete for the same customers; Call of Duty's creators have a paramount quantity of customers buying from them.
343i do not care for making their work original, fresh and new. They care for only their success and money pouring out of every hole in the CEO's body (Or highest-status EO).
I'm not saying that Halo 4 is bad or 343i are just like a cloned EA that plays fetch with an uranium bar until cloned EA (343i) are mutated enough to just listen to fans.
Well, the second bit. Kind of.
Halo 4 is still a good game, the direction of most main-stream FPS games must be considered; fast pace with more player-variables to change outcomes of combat.
343i, being a division of Microsoft (I think "division" is the most accurate word, correct if wrong), has an intention of gaining a big customer base.
Why? Microsoft games. It is likely that a large Halo-fan base will mean people will be interested in other Microsoft games.
Although this may not be an intention, you devalue one item's qualities to give it astronomical value of coin.