The short answer: both. So:
Why do Catholics often leave off the words at the end of the Lord’s Prayer?
Actually, Catholics are free to recite the prayer either with and without those words, and they do include them at some Parish Masses.
The Lord’s Prayer is unique among all prayers because it was taught by Jesus Himself.
Catholics commonly refer to it as the “Our Father,” after the ancient tradition of using the first two words of a prayer to name it, e.g., Hail Mary, Glory Be, etc.
Catholics and non-Catholics sometimes recite different versions of this prayer. Typically, the Catholic version ends with the phrase “deliver us from evil,” while the non-Catholic version goes on to include, “For thine is the kingdom, the power, and the glory, now and forever.” These additional words form a doxology, a short, flowery verse of praise appended to a prayer.
The apparent discrepancy between the two versions of the Lord’s Prayer naturally causes one to ask which way of praying it is the way Jesus originally taught.
This prayer is recorded in two of the four Gospels—Matthew and Luke. The two accounts are essentially the same with some phrasing omitted in Luke. Jesus, endeavoring to teach His followers how to pray, declares:
“Our Father who art in heaven, hallowed be thy name. Thy kingdom come. Thy will be done, on earth as it is in heaven. Give us this day our daily bread; and forgive us our debts, as we also have forgiven our debtors; And lead us not into temptation, But deliver us from evil” (Matthew 6:9-13; Revised Standard Version, Catholic Edition).
Some translations of Matthew, most notably the King James Version, produced in the seventeenth century, include the aforementioned doxology to the Lord’s Prayer. The majority of the oldest surviving manuscripts, however, do not. It should also be said, furthermore, that the doxology does not appear in any translation of Luke. Thus, it is believed that the doxology was not part of the original Biblical text, but was later inserted by a scribe in the process of translating it. Likely the doxology was initially written in the margin and eventually came to be integrated in with the main text in a certain line of manuscripts.
While the doxology is not found in the most authoritative manuscripts of the Bible, the tradition of the Lord’s prayer doxology goes all the way back to the early decades of the faith. The ancient Church document known as the Didache (did-ah-kay), for instance, which was probably written in the latter part of the first century A.D., essentially at the same time as the New Testament, includes the doxology.
The Didache is a thoroughly Catholic document—expressing beliefs about Baptism, the Mass, and Confession that agree with what the Catholic Church teaches today. The custom of adding the doxology at the end of the Lord’s Prayer, then, appears really to be part of Catholic tradition. And while it is believed the shorter version of the prayer is probably the version which Matthew originally wrote down, both versions of it are perfectly acceptable for Catholics to pray. In fact, the doxology is regularly recited by Catholics today when the Our Father is prayed at Mass.