Idk about you but I don't know the last time I played split screen halo. Maybe when I was a young teen at a party? I'd much rather have what the excellent game quality they're delivering than being able to split screen one day out of the year maybe on like a holiday. Split screen isn't as big of a thing anymore.
Some people like having they're friends over to play with them tho
But it also comes down to memory budgeting. "Some" is probably not an overwhelming amount that says we need to have this in the game and so at the end of the day, when you're entire game is very large and excellent quality, features like this can only be put in the system if they have space in the memory of the game. Similar to with ttk and the vault. We were originally only going to get 3 extra spaces until someone managed to work out 1 megabyte of memory and was enough to double the vault. The interface and UI and such for split screen features is probably too much and could have hurt the quality of the game if it was put in over something else
In halo 4 they had great graphics for single player and then they lowered them for spit screen. This wouldn't be an issue
Yes and in halo 5 the graphics and cutscene are twice as detailed and the campaign is over 50% bigger, the forge capabilities allow twice as many objects placed, there are over 4 times as many objects as before that can be placed on any map meaning each map has to be made to operate with them and the capabilities such as grouping and the 100+ new colors all take up an enormous amount of memory that has to be saved for it, and you have all of these new game modes and massive maps from warzone. Yes this would be a problem.