Of course the XB1 is capable of running 4 play split screen coop. Whether a games does or does not include that feature has absolutely nothing to do with the console, but the developer priorities/budget/time etc.
As for this thread. This is a thread with less than a thousand posts, with even less individual posters, this thread really doesn't mean much, and trying to draw any broad conclusions from it is silly.
But yes, of course it would be in 343's interest to cater to everyone they can. But again, depending on various factors, it might simply not be possible to do so. And if they did focus on including this, there would be a tradeoff somewhere else. Something you still seem to not understand.