Guess 343i relied too much on numbers. Some examples, just assumptions.
1. Infection: They saw, that a lot of people played Infection, so they decided to make it more like a Zombie vs. Human alike gametype.
2. Grifball: Same like Infection, I guess...
3. Theater: I would say they cut it off from campaign and SpOps, because they saw, that people use the theater mode, more for multiplayer/MM then for anything else. Maybe the QTE and SpOps CGI's are an other factor, why we can't have them anymore. This is a mistake, in my opinion.
4. Flag/Objective gametypes: Objective gametypes aren't played that much, like Slayer gametypes. They decided to change it, so that it becomes more enjoyable, but I and many others, too, don't think, that not being able to drop the flag is a good idea. We'll see.
I could continue the list with a few morw points, but I'm tired and I need to sleep. You most likely got my point and what I'm trying to say. What really matters are the numbers for them, I guess.