Been wondering that myself. He was something like [and this is just from memory, so I might have the exact number wrong] 15 wins away from breaking Don Shula's all-time NFL victory record for head coaches.
As a Dolphins fan, I expected [and dreaded] BB latching on with some other team, and slogging through 2 to 3 years as a head coach, just so that he would break that all-time mark. That is 100% what I expected him to do, if for no other reason than to break the record.
I seem to recall that last year, he interviewed with a team [might have been the Panthers, but could be wrong about that] about a head coaching position, but the GM didn't vibe with him. I think that's why he ended up in the studio last year.
But to "waste" a year coaching at the collegiate level doesn't make any sense to me, at his age, with the record within legitimate striking distance. Maybe no NFL team would hire him, due to his personality / how difficult he is to allegedly work with.