Why do Protestants end the Our Father with “For thine is the kingdom, and the power, and the glory, forever and ever” but Catholics don’t?
Because it is doctrinally sound, Catholics do pray what is called the Final Doxology (“For thine is the kingdom…”) when praying the Our Father at Mass, but it is separated from the Our Father by an interspersed prayer (“Deliver us, Lord…”) in order to acknowledge that it does not properly belong to the Our Father as it was given to us by Christ.
The doxology originated as a gloss added to an early manuscript by a copyist. Later copyists mistook the gloss as part of the Scripture text and incorporated it into the Bible by mistake. It would be analogous to one student scribbling a note to himself in a textbook and later printers mistaking the note for text and incorporating the note into future editions of the textbook. Protestants who relied on these erroneous manuscripts of the Bible when compiling their translations of Scripture perpetuated the mistake into their own Bibles. Catholic translations of Scripture do not include the doxology in the Our Father because biblical scholarship now concludes that it is not original to Scripture.