2 Bo has been an assistant on his father's staff for years. He knows the system, he knows the kids, and last time St. Marys brought in an outsider they immediately went from playoff team to 0-10. They're a Wing T team and that's not changing.
Call it nepotism if you wish, but he's the best man for the job.
But just because he knows the system, knows the kids does not guarantee success. Bob Priddy was good defensive coach (And a great person) on Coach Baughman's staff for more than 20 years but his head coaching stint did not start or end well.