my guess (spoiler alert: i don't have any insidery info), is that frank felt like although he knew his ad was here for the longterm (despite false rumors that his ad was licking his chops at any job outside of kansas), he just couldn't commit anything more to the university that gave him his first break into the head coaching scene and instead of getting burn out, he moved on.
who knows. time to move on like frank did and embrace the moment, go cats and go riley county america, home of the cats.