Its simple, Halo Reach started this downward division. Not saying that from Halo 2 to 3 didn’t have its effect, it did but the big divide was Reach. Think about it, if Reach never came to be Halo 4 may not have been influence to act like CoD or w/e we think it being right now.
Since the TU update which goes hand-in hand with Sept. 14th Reach’s gameplay, along with ZB, has cause a massive division on what to prefer as a good gameplay style. Halo 4 which just been revealed leds to some hints about sprint and single shot weapons with bloom possible. The progression system, perks, and this whole red vs blue fiction which why even create when its been around since Halo 1 thanks to roosterteeth, just deviates it too much.
In addition more and more playlists gametypes are being created and expanded more so than Halo 2 ever had and was on par if not more than 3 which again divides population. It sucked when their was low population with the divide population in playlists and TU affecting people’s motives in playing this setup and old vice versatility (I guess) just was not good in any sense.
If Halo 4 is for change it may not be in a some what good way, Halo 4 took the Reach/CoD route and decided to not expand on the equipment use Halo 3 did, if anything Halo 4 took from 3 was the BR, movement in some manner, and story continuation.