Many private schools lack the resources to stay cutting edge on their endzone cameras. Typically, when a successful private school coach takes a job at a public school, he will not be nearly as successful because he isn't prepared to utilize his new state of the art endzone camera to its fullest potential. That's the best explanation for why private school coaches usually can't replicate their success at a public school.
Notable exceptions would be Kevin O'Shea at North Central and Dick Dullaghan at Carmel and Ben Davis. Both of those coaches were already up to speed on endzone camera culture when they made the jump from private to public.