Historically the Catholic schools don't bring in a large gate. They don't travel well. Public schools bring in huge amounts of fans including alums. By having move Catholic Schools playing Public School games, you get to create the David v Goliath matchups!
I think the problem is it ends up being a Goliath vs. Goliath match up too often.
If you were from another state and you saw a Super 8 program where a school like Reading High with approximately 650 boys or Hingham with 500 boys all from one town which is basically one ZIP Code was expected to play against a team from a high school like BCH with their 1,600 boys and a team consisting of kids from 20 different towns/ZIP Codes you would think the system is flawed as hell.
Why not a Public Tourney and a Private Tourney and either a Public vs. Private winner game or a Public All-star Team vs. the Private winner? Talk about a huge draw on both fronts!