If you're throwing to a bunch of guys that either can't catch or can't get open, your stats are gonna suffer. And if they can't get open against inferior DBs, that makes it even tougher for Allen to up his completion %.
I would think it's fairly obvious that Mayfield, Rosen & Darnold had a better cast of characters on offense than Allen. Let's see how many offensive players are drafted out of OU, USC & UCLA vs Wyoming.