1. With a great team coming back it was vital for Xavier to bring back a coach they can relate to. Mack is familiar with the players and has built relationships will all of them. For example, Kenny Frease was contemplating transferring until he heard of the Mack signing. Mack was the main person that recruited Frease to Xavier. Yes, Derrick Brown may still test NBA waters but with Mack at X they won't have to worry about players transferring.
If Xavier decided to hire a guy like Darrin Horn then players may have transferred and X would have to re-build. Not to mention if the team started on a downward slide because lack of talent how would this help recruiting? Incoming recruit Kevin Parrom may have tried to opt out of his scholarship if Mack wasn't the coach, next week Mack is visiting him to make sure he's happy.
2. Mack is dedicated to the university. No other coach in Xavier history has went to St. X, played at Xavier as a captain for two seasons, and then went on to be head coach of the basketball team. I'm not saying that Mack is going to jump in and have instant success, but he certainly will have the tools around him to get started. People were calling for Sean Miller's head after his first season at X after they missed the NCAA tournament and looked what happened. Also, the 5-year contract means nothing to me because college coaching contracts are pointless nowadays but Mack could be the guy that sticks around for awhile, I hope.
3. The players on the current team are used to his style of play. I'm sure they'll need to learn some new plays but the offense and defense that Mack puts together on the floor is going to be close to what Miller put out there. Mack is more of a defensive minded coach but at least the players won't have to learn whole new offensive and defensive schemes.