Re: Halo 5 sold less than 1 mil. copies in the U.S
I think I'd chalk it up to competition. You have Call Of Duty, Starcraft, and Fallout all seeing releases in a similar time frame. Digital sales is probably a factor too. That and the fact that the Halo story is just kind of stupid now. MCC disaster. And then… Destiny.
All of these taken together probably explain it.