Any chance this has to do with them dropping the Split-Screen Co-op which was a staple of the other Halo's? Personally, it's the main reason I haven't picked it up yet. I've always enjoyed playing Halo co-operatively with a friend over or at a friend's house. Perhaps dropping it turned off more people than they expected.
"It really has been heartbreaking to hear all these stories about players who were disappointed because they couldn’t play Halo 5 with their children, or siblings, or spouses, or friends because of the lack of splitscreen. This is now an age where you have to double-buy Xbox Ones and TVs within the same household to make that happen, or you have to tell someone to stay at home and play on their own console. That, quite frankly, sucks. It sucks enough where some people have said they haven’t bought Halo 5 because of that, and I can see where they’re coming from."
http://www.forbes.com/sites/insertcoin/2015/10/31/halo-6-might-have-splitscreen-because-of-halo-5-backlash/