The biggest advantage may be that the GC at the catholic schools know how to create the application that will get their students into the catholic schools. What does BC look for? Does Dayton or Santa Clara want letters of recommendation that highlight leadership in church activities, that talk about service, that talk about sports and school activities? The GC have connections with the admissions officers at the colleges. The Catholic high schools host college fairs or presentations by these colleges.
Catholic high school students also apply to the catholic colleges in much higher numbers. They might be legacies. They might be athletic recruits.