I got Halo 3 (used on day one!) and it was great. I went back and got Halo 1 and 2 and they were great. I got Reach and it was great. I went back and got ODST and it was okay. Then, I got Halo Wars and it was "meh". Then I got Anniversary and it was decent. Then I got Halo 4 and it was pretty good. And then, I got burned out.

No split screen was the kicker. I usually just play campaign co-op.

I did get The MCC a couple of days ago and it is superb. I'll probably give Halo V a try, eventually.

It's been the weirdest phenomenon. I surely thought those who were still standing behind Xbox One would be the people that would buy the game in droves, I suppose not. I can only guess...

1. Following up Halo 4 and MCC tainted the waters
2. Shooters like Destiny and CoD and such are eating its lunch
3. The press seemed lukewarm to hype it like past Halos

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. 

Destiny.

Bungie left.
i343 is good but they are not what some Halo fans wants... to be fair some Halo fans are Bungie fans and moved to Destiny.

BTW Halo 5 is the less Halo game already made... it is a realy great FPS but it not a good Halo.

The milking. The constant milking of a franchise will cause it to lose its appeal. MS literally have a whole studio (games and visual media) dedicated to making Halo stuff.

What I noticed about Xbox gamers (and this may be totally inaccurate) is that we really don't care about Xbox.  My loyalty to Xbox is pretty much like my loyalty to a microwave brand.  If does what I want it to do but there's no emotional attachment.  Not like I have for Nintendo or against Sony.  M$ is just kind of there.  I knew tons of people that bought an Xbox 360 but i don't think i know anybody that cared about it aside from "it has good games". Halo is a great franchise but it's never been like Zelda, Mario, Uncharted, etc. At least not for me.  You have people who read Halo books and jerk off to Master Chief fan art, though.

*bonus* I buy Nintendo because I love them. I buy Sony because, love them or hate them, you kind of have to have a PlayStation or miss out on some great games. I just buy M$ because they do certain things way better than the competition. 

The fact that many have moved over to ps4.
Imo third party games like COD and gta seem to be what's selling consoles more than exclusives these days. And people buy cod and gta on the console that most of their friends have. People will buy exclusives for the console they own, but when that console is ps4 they won't be getting halo.

I think that the majority of Halo fans have begun to see Halo for what it has always been,since most Halo fans were teens/young adults when Halo first released.The games may not be as good as when Bungie were creating them,but Halo has always just been hype and the hype has now finally died.MS also relies on the game too much and people have moved on to better games.

This and the fact other long lasting franchises will decline with time regardless in the end, if it happened with Halo it will likely happen with others.

I do reckon 343 should work on something completely new whether it be a shooter or something else.

Install base,too bad we don't have Halo 1 and 2 first week or month sales