I have no inside information on this. I'm just throwing out my leading theory.
Given that Urban Meyer is going to have a say in the coaching search to name his successor and he is going to have a role in the program, he is going to want a guy he trusts and who understands his system -- but someone who will never challenge Meyer (nor make Meyer feel marginalized). He will also want a guy with previous head-coaching experience, preferably at a high level.
That's Dan Mullen.