Simple business cycle.
Managers are built for certain phases of an organization's growth. There are effective managers who do great in the start-up phase, others that thrive in the growth phase and so on.
Marrone's approach was needed to change a culture and set a structure. He was a low-risk guy who instilled an approach that is needed to build an atmosphere conducive to winning. By nature, Marrone is not going to take many chances - many here questioned a lot of his play calling, etc. But SU's football program couldn't afford to take many chances - which is why he was exactly the guy for the job.
As kcsu said, Shafer was a great hire for this time and the foundation is in place. He has the ability to take chances that the program couldn't before, and has the temperament to do so.
I'm not sure Darryl Gross understands this type of thing - I honestly believe he sort of fell into this by blind luck. But sometimes it's better to be lucky than good. So kudos to him for making the correct choices after a miserable one in GRob.