No, it was because Halo sold a lot of copies and the industry is full of cargo cult design.
Also, while I may not have played on the upper difficulties (and I have never found the idea that bumping up the difficulty improves anything to hold true) I have watched other people play on them and it was still heavy on the ol' assault rifle.
But even if you have to mix it up more than I am making out, it's still turning weapon choice into something the level designer is largely making for you. I don't think that's a good design decision.
Anyway. Even if you for some reason like two-weapon limits, I hope we can agree that Wildlands' system does not accomplish the ostensible goals of that particular design choice while making free access to all the guns unnecessarily cumbersome. Which was my original point.