One of the consistent things every Halo game has had is co-op play. And while it simply started as clones of Master Chief running around in 2 player co-op Halo, the game progressed to integrate it into the story with Halo 2 as the Chief and Arbiter were the focus of that campaign. And with four player co-op being added in Halo 3, it changed the benchmark for the series.
Now with the Master Chief Collection, it looks like there's a chance the first two games could get 4 player co-op added to it.
Answering a player question on their official November update blog about adding four player co-op to Halo 1 and 2, Community Support & Engagement Coordinator "Postums" noted that although the team "loves" the idea, it's "further down the list" in terms of priorities. Postums hints that although it can be done, there are more important tasks to get to, like updating co-op netcode or "bringing lost content to the [older] games," or other "global updates."
In fact as you look into the story further, there was actually some loose attempts to do this inside of 343, but it appears there are some game breaking things that can occur. So for a top tier developer to actually release that support, it would require a level of polish.