I think it was 95% Saban being a prick. A chance at turning UM's O around and working with Diaz was the other 5%.
Like many posters have already said, if he was in a rush to be a HC, the smarter play would have been to stay at Bama as OC and coach that ridiculously talented and high powered offense for one year and then be a HC once Tua and Jeudy leaves. Easy money. I think he is here for the long run. Maybe he would leave for an NFL staff gig but not seeing him rushing to be a college HC.