"Fr. Gruner gives more details about the coming chastisements (19:20). He explains that if the Secret was released, it “would stop the changes of Vatican II in its tracks: immediately”."

This would explain why the Third Secret which was supposed to be revealed in 1960 wasn't, and why Pope John XXIII railed against "prophets of doom". Given the collapse in the practice of faith the "prophets of doom" seem to have been vindicated.  Reportedly J23 fainted when he opened the secret.