I think there is a possible explanation to this. I remember, there were rumors/reports of an early offseason/OTA battle between Morris and Tolzien for the 1st team snaps. I didn't pay much attention, but looking back now, I see a chance that that that battle decided everything. They knew Luck won't be available, so they might wanted to choose one guy early, who will receive the majority of 1st team reps all offseason. Maybe the coaches thought that there is not much difference between the two, so it's more important to pick one guy and make him prepared as much as possible, than letting them compete by the price of sharing snaps.
Maybe that's it. Maybe Morris lost that early battle, which decided his faith. I know, it's maybe not fair treatment, but it's a business, they do whatever they think is the best for the team. And, then came the new guy, Walker, who pushed Morris even further down. Not because he was better than Morris, but because he was PS eligible. The coaches knew that they can put him on PS, so they gave all 2nd team snaps to the guy they can keep, rather than to a guy who they knew they have to eventially release. (Because they wont keep 3 guys on the active roster once Luck comes back.)
And here we are in september. Morris's fate was decided long ago, he was not in consideration at all. It didn't matter how he played. Maybe, if he played like a world beater, next big thing ,they might have changed their minds. But he didn't. He did OK, he was a bit better, but the difference wasn't at a whole different level. The difference didn't worth the change.
Then came yesterday, the Brissett trade, which was a clear indication, that even if Tolzien won the offseason battle, they are not sold on him neither. Not any more. He'll be given the first 1, 2 maybe 3 games - because he was the guy who practiced with the 1st team all offseason -, then they say farewell to him, and go with Brissett. And, despite Walker sucked, they give one year to him in the PS, and see how he develops.
I'm not saying this is how everything happened. This is just one screnario I can imagine as a reasonable explanation, instead of "the whole coaching staff is soo stupid that they don't see what even an armchair fan can clearly see."