Why is change good?

I spend a lot of time reading these forums, and I often see comments along the lines of “The game needs to evolve or it’ll get stagnant.” and “Halo needs to change to keep up with the player base”. My question is, why? What evidence is there that this change improves the experience and brings in players?

The case in point is obviously the CoD series. This is a series that is extremely popular, and its fanbase is ever growing. This is despite the fact, since CoD 4 was released in 2007, very little has changed in the series. So in an environment where games apparently require change and evolution to survive, why is a series like CoD, which only makes minimalistic changes if any, doing so well?

I’m not suggesting that 343i change nothing and simply re-skin a H2/H3 game, I’m simply asking the question why change in the series is suddenly so important. The halo series went from having minimal changes from H2 to H3, to suddenly drastic changes for Halo Reach and then again for Halo 4. I also think it’s worth mentioning that Halo really dominated the FPS genre in the H2/H3 era, and has started slipping since the release of Halo Reach.

tl:dr
There seems to be a lot of evidence that small changes in gameplay over a series, rather than drastic sweeping changes, are more effective. Why do people claim that drastic changes are needed to avoid stagnation in a series when the evidence suggests otherwise?