A not-great 'battle pass' unlock-style system that required tons of grinding, halo reach's arguably, but generally agreed upon weaker multiplayer/maps and sandbox, and the various bugs/glitches and what have you on PC probably soured people from sticking with the game, rather than Halo as a whole.
I'm curious to see what would have happened were this 2 or 3 launching first - but IMO even if the latter 2 titles cause a bit of a comeback, the tide's kinda passing them up.
Anniversary isn't gonna be out for several more months (presumably), and by the time 2 is out, we'll be in next-gen - and then you either cannabalize Infinite, and/or people don't care.
(Granted, most probably bought all of the titles already in the MCC, but the point stands)