The difficulty with trading for Rosen is it goes against the human nature these GMs have of "being right". Everyone who passed on Rosen or was in trade up range but didn't trade up to get him spent last season saying, "See, I told you he wasn't worth that pick". Suddenly tossing a draft pick to the Cardinals for him requires a bit of humility and out-of-the-box thinking for how that can be spun INTERNALLY. "Wait, suddenly he is good enough to be on our team" is a question that every GM that falls in this category will have to answer confidently that trades for him.
This is why I could see a team at the back end of the draft last year, who didn't have a realistic shot for him trading for him, getting him and making him a backup. He would benefit from the learning under a vet and would be put in a much better position to succeed. He actually makes a whole lot of sense in a place like Pittsburgh.