He's certainly highly qualified and a great coach. But you don't know if he can design and implement a defense and whether he can call plays. He may be great at all of that but it is a risk because he's never done any of it before. I tend to think that I'd rather take the chance at greatness rather than the safer choice in Staley, but I can see why Moore might prefer the safer more experienced option.
I think we just have to trust that Moore will know if Parker is ready for a DC job.