Thats because the pathway for CHS players is now to get a scholarship at a GPS, CAS or ISA school.
A gross over-simplification really but has only occurred for certain boys because:
1. Wednesday afternoon CHS zone rugby competitions have almost disappeared - nobody to coach and often below critical mass needed to run team/s
2. Club junior rugby has also contracted at the same time, leading in part to fewer boys with rugby playing experience going to their local state high school
3. The establishment of sports high schools has drawn sporting talent who play both league and union and the teachers at these schools have a league orientation (one of the sports high schools forfeited a Waratah Shield semi final a couple of years ago because it clashed with a league game)
4. Lack of interest and resources among staff and leaders at state schools - they are just required to provide sport so that the sports which are (a) easy to coach and (b) are supported by governing bodies are played
EDIT: You'd find that most of those kids would stay at their local state school if their was a half-decent rugby program their (which used to exist not so very long ago)