Is this the case of not having enough market-share?
HALO 2 was on the OG Xbox. I doubt it's install base. Install base never stopped Halo before because it shifts consoles if needed and/or has a high attach rate.
This is much more likely to be Halo 4 failing to be "sticky" with majority for MP, a narrative that now excludes those that haven't bothered with EU (they may a big mistake with this IMHO) and lingering effects of MCC.
If word of mouth on MP is good enough it might improve but the franchise took a big it with 4 and MCC.
I was always there day one for the SP which I enjoyed in its cliched Science Fantasy way and then moving on to coop/MP.
Not so with 5. I have no interest in all the blue team, Locke whatever stuff and I've found the way the narrative has gone boring now, over focusing on Spartans, politics and over aggrandising its C grade SF story.
Coop with buddies is gone and while MP is good the whole isn't enough to make me feel I need the game enough to get an XB1.
I've tried it at s friends and dropped SP as it just interest me now. MP I thought was very good particularly at 60fps and that's what they need to focus on short term I think.
Personally though I think the whole narrative approach needs s major rethink and reboot.
After Halo 3 I think many (myself certainly) thought we were going to see. New cycle with new enemies, a new place in deep space and a sense of a soft reboot). Instead Halo 4 started with more covenant (yawn) introduced new foes who were dull and it turned out I needed to read books to even get what they were after (no thanks) and then it screwed the pooch being overly self referential. MP that didn't feel like the Halo MP I wanted really was the cherry on top.
Halo 4 wasn't bad as such, it was very solid really. But it wasn't what I (and I think many) wanted. MCC then eroded confidence in 343 to deliver robust code. When Halo 5 marketing kicked off it felt like it was for the hardcore into the EU and not for someone like me who only plays the games.
In fact outside the MP that's how I would sum up H5 at this point. Designed for the smaller EU hardcore not the larger games only demographic.