A lot of the reason he wasn't fired was because:
1. Larry Templeton was our AD.
2. We were on probation.
Let's just be thankful 2007 happened. Not many years where we can say that we beat Alabama, Ole Miss, and won a bowl game. I don't care that we had to return a bunch of INT's and punts back for TD's and the Liberty Bowl wasn't Super Bowl 24 offensively- we still won the games. And we beat an Auburn team that I think ended up being ranked at the end of the year, and a pretty decent Kentucky team on the road and even managed to beat Tulane in the Superdome.
If you look at the bottom line that technically ranks as Dan Mullen's third best season had he been coaching the team.