First, a lot of the blame falls on the Xbox One rather than directly on Halo 5. Many of the consumers in the market for a home console have either bought a PS4 already or they're leaning in that direction because it's the clear market leader.
There is also the fact that the Halo occupies a space that is rather crowded. There are plenty of other popular choices for multi-player shooters out there.
However, I think a lot of the responsibility does fall on Halo 5. I noted months ago that the videos for Halo 5 were nowhere near as epic or inspiring as Halo 4. I think the switch in narrative resulted in a loss of mystique for the series. Nothing shown before launch really pulled people in and made them care, beyond the normal, "it's another Halo game," impetus. There is kind of a been-there-done-that vibe and it doesn't feel particularly important.
If they want the franchise to regain some luster, the next game probably needs to be a lot more epic in one way or another. Right now, it's in that, "the adventure continues," area.
