It’s a lot of factors. Here are some IMHO, in no particular order:
- More lay people teaching than Priest, Nuns, Brothers and Friars so your cost in staffing has gone way up not to mention other overhead costs.
- Aging facilities and the need to upgrade and not having the financial resources to keep up.
- Less financial support from either the Diocese or the religious organizations that run the school
- Less people going to church leading to a reduction of people having the desire to send their offspring to a religious institution
- Increased tuition which parent have a need to pay in full without financial help that might have been available in the past.
There are at least three here on LI that have closed in the last ten years (not even going into the whole Maria Regina, St. Agnes, Holy Family, St. Anthony’s from 30 years ago) including one pretty prestigious all girl’s school.