You are right that great coaches usually don't get blamed for much, but the key difference is that if you are proven then you get the benefit of the doubt during a down season. Meyer won a SEC title and national title in 2006 so he got a pass for 2007. When a coach is proven we know what they are capable of so when a bad season comes we can expect for them to rebound. With an unproven coach we have no idea if they are capable of being a good coach, so if the results don't come people understandably doubt them. Zook and Muschamp won zero national titles, let alone SEC or even SEC East titles plus they had average records so they get no benefit of the doubt from fans.