Spartan Ops failed to keep people playing Halo 4
According to a case study on Raptr, player engagement for Halo 4 fell off more quickly than Halo: Reach. A large reason for the poor user engagement is the failure of Spartan Ops -- a series of episodic missions, similar to Call of Duty's Spec Ops missions, that was free to anyone with Halo 4 and an Xbox LIVE Gold account.
The Spartan Ops episodes released weekly, and the hope was that it would keep playing. In reality, five months after Halo 4 released, Halo: Reach actually had more hours played per week, according to Raptr's numbers. There were spikes in daily playtime when a map pack got released, but there were no residual effects of Spartan Ops episodes.
Neither the DLC or Spartan Ops kept players playing consistently -- something that weekly content should've remedied.
Personally, after I beat the campaign, I played the multiplayer pretty hardcore for about a week and a half. After that, my playtime waned and I moved on to other games. That's quite normal. I didn't return for any of the DLC, mainly because I got my value out of Halo 4. But for Halo 4 to drop below Reach, something must be wrong with the approach to multiplayer. Maybe 343i needs to shake things up a bit with Halo 5.