Because most other Bibles tack this phrase on the end of the Lord's Prayer
For thine is the kingdom,
and the power,
and the glory,
for ever and ever.
And the WTS teaches that is a spurious addition.
So if you are handing out the Lord's prayer from another translation, leave that off. They might take it.
*** w64 4/15 p. 231 What Will "God?s Kingdom Come" Mean to You? ***
Contradiction was brought into this prayer when some religious copier of the Holy Scriptures added to the prayer as given in the Sermon on the Mount the following words: "For thine is the kingdom, and the power, and the glory, for ever. Amen." (Matt. 6:13, Authorized Version, Martin Luther German Version) As a result hundreds of millions of religious people in Christendom have for centuries recited the prayer with the addition of those unauthorized words as a conclusion or doxology. Seemingly they have never stopped to think of how at the beginning of the prayer they could recite the words "Thy kingdom come" and then in the conclusion of their prayer say: "Thine is the kingdom." If the kingdom was already God?s, why should they pray in the same prayer: "Thy kingdom come"?
This shows the foolishness of adding something to God?s inspired Word with the idea of trying to improve it or fill it out.
*** w63 3/15 p. 167 What Does the "Lord?s Prayer" Mean to You? ***
In the Authorized Version the words, "for thine is the kingdom, and the power, and the glory, for ever. Amen," are added at the end of Jesus? model prayer. These words, however, are not found in old manuscripts such as the Sinaiticus, Vatican 1209, Codex Bezae and the sixth-century Codex Palimpsestus Dublinensis. They are, therefore, evidently spurious and are left out of modern translations.