Looks like not having split screen in Halo 5 eroded fan support. 343 Studios will be including Split Screen going forward for all first person shooter games. Unclear is whether they intend on patching split screen into Halo 5, but my guess is that would be unlikely.

Related article:
https://arstechnica.com/gaming/2017/02/halo-split-screen-combat-is-coming-back-and-its-here-to-stay/