Simple way to say it. People like the same game over & over again with new maps and very little change because they are humans. Humans tend to hate and avoid change when possible, and often reject it when offered.
Halo 1/2/3 were all very similar, which while they added new things, made them more boring to me as time went on. However, ODST tried to change that a bit, putting us in a roll as an ODST rather than a Spartan, Reach changed that by adding alot of different things. 4 has now changed it once again, we have alot of things taken from other games, but for people like me who like those other games, this is amazing! I like CoD, I like Battlefield, I like Halo 3 and I love Halo 4 because it combines those games with my good ol Halo 3.
If people wish to continue playing the same games over & over again, so be it, I like that games are changing & evolving, I think Bungies best move they ever made with Halo, was with Reach when they attempted to change the game up for once. They took a large risk, they could very well turn people against Halo, or draw even more people in, it seemed to have negative effect in the end but the fact they took that kind of chance is what made me happy, very few companies do that, alot of them just recycle old games & resell it.
Look at CoD, they make the same things every year with some small changes, the campaign is nothing all to interesting in most cases, its really just a remake. I like those games, but I still think thats all they are, Battlefield is much the same. Halo was on that same path, but I am happy, you could almost even say I’m proud, that its taking a turn & trying to do what others do not have the courage to do, making it a real new game so its not just me buying Halo 1 part 4. The difference that people do not see is that when you look at CoD & that kinda game, people stay, they follow it, they continue playing it & buy the same thing every year. As I said, Halo may have lost people with Reach & 4, but at least it doesn’t feel like it was just another yearly installment thrown at me with the same basics to the game, with minor changes.