When the SCC was formed, it was mostly LL and L class schools and after a few years of getting whitewashed in many sports both Seymour and Derby left for the NVL, both of whom were founding members of the old Housatonic League.
Other two nearby leagues, FCIAC and SWC are also mostly LL and L schools.
The NVL, with mostly L and S schools, is challenged by demographics (shrinking population in many towns and below-low average income) that has made it hard for many of them to compete, especially in football. As it has worked out, of the large schools, only Nagatuck has been consistently above average in football, while the other competitive programs are S schools in Ansonia, Seymour, Oxford, Woodland, and Holy Cross (Oxford used to send its kids to Seymour while Woodland sent its kids to Seymour and Naugatuck). It's also wothwhole to note that that St. Joe's in Trumbull, Fairfield Prep, and ND West Haven all 'recruit' from the Valley, too. It's a tough spot.