I think it comes down to quality of assistants in most cases. His best recruiter by far was always Cale Gundy, and that gift predated him by two decades. Bedenbaugh is an excellent coach and evaluator, but is not a headhunter on the recruiting trail. I also wonder if Riley's skills as a coordinator make it hard for him to identify and teach those things to other coaches on staff.
It goes back to why transcendent athletes are often poor coaches. It's impossible for a Wayne Gretzky to teach someone to play like he played, and he has no idea how to maximize the talent or adjust around the skills of those around him outside of being a player himself. Extend that one level higher to transcendent coaches. Imagine Lincoln giving advice to a young DBs coach or DC like "Just perfectly remember what worked best in this situation the last twenty plays and call a perfectly disguised coverage to bait the offense into going to your strength." Great, but what if that coach doesn't have the eidetic memory and intuition about tendencies that Riley has for the offense??
Now Riley's lost the teachers, the evaluators, the planners, and the trainers that all build upon each other's work to make a complete team. He's relying on himself and another generational talent to make up for it all, and if QB1 isn't absolutely note perfect every snap, then Saturday happens.