I wonder, myself, if he thinks he's not ready to be a head coach yet, and wants another year. Or wanted a future franchise QB in place wherever he went.
I think he wanted a big bag of money and control and owners were shy of risking that on a bright guy with no HC experience.
I also suspect he saw what job openings remained and didn't like them.

Between those two things he can spin it as he wanted a ring -- it would be his best chance at one in the short term, anyway. He still has the OC job, so he had nothing to lose by kicking the tires on HC openings.
Personally, this is good for DET and bad for the rest of the NFCN. I think he's a really good OC. No idea if he can be a HC though... a lot of guys get over their skis and stink as a HC, or they are good but it's masked by having a garbage roster and impatient owner.