You know, this is something a lot of people here overlook. What has Halo done in the last 10 years that no other shooter was doing? Splitscreen parties going into Matchmaking. COD doesn't do it. Battlefield doesn't do it. What has changed in the last 10 years? They stopped supporting splitscreen adequately. I've played splitscreen for years now. You know what boosts population numbers? 4 players on one box.
I can't say I blame 343 (or any other dev for that matter). You're working with old hardware and trying to optimise it for better graphics and whatnot. Rendering 4 viewports is always going to be a significant challenge but it is crushingly sad to not really be able to play anymore. It's supported but Krow and I are quite immediately at a disadvantage in almost every way possible.
The framerate goes to shit but the EXP system goes out the window. The reticules are weirdly sized. The XBL notifications actually block the bottom player's screen quite considerably (including the reticule).
Whatever about how relevant Halo is in 2013 or how a 5 shot BR killed the game, the lack of splitscreen users is surely a contender for Most Significant Factor.