I'd add Halo 4 to that list. I think people just lost interest in the series over time in favor of new ones that have popped up.

I didn't really like the story in 4, 5 or infinite all that much. I honestly don't like most of the multiplayer maps in the newer games either. I don't think the new games are terrible but ever since Bungie left they don't have the same charm to them. Like Halo 2 seeing the scarab for the first time, or the different locales on Halo you'd travel to in order to unravel the mystery of the ring. In infinite you go into a bunch of nondescript towers as Cortana speaks gibberish and the brute guy keeps monologuing at you repeatedly. It was just obnoxious.