The Prayer to St. Michael was part of the Leonine Prayers that were suppressed in the 1960s, I believe by Pope Paul VI. He didn’t give a reason, the Vatican document as I recall just said “The Leonine Prayers are suppressed.” I remember my mother saying that even back in the 60s people complained that the prayer was too scary for children. Other people have complained that the Leonine Prayers were “adding on” to the Mass.
In the last couple years since I started going to many Masses at many different churches, I have found some where the pastor already has everybody saying the St. Michael Prayer, or in some cases the St. Michael Prayer and the Hail Holy Queen (which was also a Leonine prayer), either after Mass or during the announcement time before the final blessing. Traditional parishes tend to say them both after the final conclusion of the traditional Mass (with the Last Gospel etc.) Now that the Diocese has ordered them to be said, I am hearing them almost everywhere. It’s rather interesting to see who in the congregation already knows the St. Michael Prayer and just belts it out and who is frantically looking for the missalette that has it on the back cover.