Duce is in a tough spot. He's regarded as a great 'leader of men' type coach, but not an X's and O's type guy.
That makes it hard to hire him for a coordinator position. You either hire him as the HC or same thing he's doing (RB/assistant HC).
I know many of the players love Duce and wanted him to get HC looks. The issue I think that came up was he doesn't have a plan to build a diverse coaching tree beneath him. He's a great rah-rah guy, but after that he's not planned how things will look in 3-5 years. Totally understandable for someone who's never been a HC, but that's a skill he'll have to develop in time.
I will add, I'm not sure where he fits with Dan Campbell. They are both rah-rah guys, so eventually you'll need some substance people to fill those gaps.