I'm sure he's not a bad coach. Just fell out of favor here.
Also, interviews are a good opportunity to also learn what went wrong, what they think about things, maybe even learn about the Saints. The Bronco's wouldn't mind learning a bit of our offensive trickery. Any coach on the team is at least passively aware of what we do.
There's obviously something about him that makes him a desirable coach. At the end of the day it's probably not the worst thing for him either; staying too long in one place can make a lot of people stagnant.