Why don’t Catholics add the “For yours is the Kingdom…” prayer at the end of the Our Father like Protestants do? Where did it come from?
The conclusion of the Lord’s Prayer that you refer to is called a doxology, which is a prayer of glory and praise. This particular doxology we do use in the Mass. After the Our Father, the priest prays the prayer that begins, “Deliver us, Lord, we pray, from every evil…” and the people respond with the doxology. The doxology is not found in the Bible, but it is very ancient. It was first found in The Didache, also called “The Teaching of the Twelve Apostles,” which dates probably from about the year AD 90. It was a letter of instruction to early converts to Christianity. Protestantism dates only to the sixteenth century, so the doxology is actually a Catholic prayer. Some scholars and the catechism (#2760) indicate that the doxology was connected to the celebration of the Eucharist and was not used apart from it.
Remember, both the Lord’s Prayer and the Eucharist were part of the “mysteries of faith,” things taught only to believers. Ironically, a remnant of a prayer used during the celebration of the Eucharist has comes to be used widely by many Christian churches that do not celebrate the Eucharist! Truth is indeed stranger than fiction!