People are saying because it released near CoD, Battlefront, and FO4 but idk about that. Halo 4 managed to sell really well even while being surrounded by CoD and Battlefield the year it came out. It could just be Halo is on the decline. I still think Halo 5 will end up with 7 million LTD.
Edit: Also yes, Halo 4 and MCC leaving a bad taste in the mouths of long time MP fans of the series definitely hurt sales.









