This game is doing about as well as can be expected especially after last year's MCC. Additionally, it's not like it hasn't had extreme competition, Destiny, a feature rich Black Ops 3, Battlefront at the pinnacle of Star Wars hype.
Additionally, a lot of what also keeps players around just isn't here so far. I mean BTB was the most popular mode of the last 2-3 games and they only brought it back like last week with 4 Forge maps. There's no campaign theater for speed runners, there is no campaign scoring for people who like to replay the campaigns, no things like rally points to skip all the cinematics and walking to get to the meaty fight sections, there is no Forge, there is no file browser or file share, and at launch, there were like 6 maps in slayer, still only 3 maps in Warzone.
There is no firefight or Spartan Ops
The upcoming DLC is supposed to be Rig and Fathom remixes supposedly according to people who said they saw it in a Sprint, so I mean, as good as the game plays, it's just so limited feature wise.