They were primarily guilty because they were primarily the ones running the schools, so of course proportionally they would have the most victims.

Had the schools been run primarily by any other religious group, they would have had the most victims, too.

Any institution which gives disproportionate power to adults over children, without proper oversight, is likely to foster abuse, and then have the institution try to ignore it, cover it up, and deny it.

Even Buddhists: http://nltimes.nl/2015/05/25/buddhist-monks-sexual-abuse-revealed