Simple. While a majority of manuscripts do contain it, some early ones which are judged to preserve a more better text do not have them. The NET footnote
for the verse says:
Most mss (L W Θ 0233 Ë13 33 Ï sy sa Didache) read (though some with slight variation) ὅτι σοῦ ἐστιν ἡ βασιλεία καὶ ἡ δύναμις καὶ ἡ δόξα εἰς τοὺς αἰῶνας, ἀμήν (“for yours is the kingdom and the power and the glory forever, amen”) here. The reading without this sentence, though, is attested by generally better witnesses (א B D Z 0170 Ë1 pc lat mae Or). The phrase was probably composed for the liturgy of the early church and most likely was based on 1 Chr 29:11-13; a scribe probably added the phrase at this point in the text for use in public scripture reading (see TCGNT 13-14). Both external and internal evidence argue for the shorter reading.
A bit of an explication for the funny little letters:
L -
Codex Regius (8th century)
W -
Codex Washingtonianus (ca. 400)
Θ -
Codex Coridethianus (9th century)
0233 -
Uncial 0233 (8th century)
13 -
Minuscule 13 (13th century)
33 -
Minuscule 33, aka Codex Colbertinus 2844 (9th century)
sy -
Syriac
sa -
Sahidic
Didache
א - Codex Sinaiticus (ca. AD 330-360)
B -
Codex Vaticanus (ca. AD 325-350)
D -
Codex Bezae (ca. AD 400)
Z -
Codex Dublinensis (6th century)
0170 - Uncial 0170 (5th-6th century)
pc lat - A few
Latin
mae-
Middle Egyptian Matthew (4th-5th century)
Or - Origen (c. AD 185-254)
From wikipedia:
The doxology of the prayer is not contained in Luke’s version, nor is it present in the earliest manuscripts of Matthew, representative of the Alexandrian text, but is present in the manuscripts representative of the Byzantine text. The first known use of the doxology, in a less lengthy form (“for yours is the power and the glory forever”), as a conclusion for the Lord’s Prayer (in a version slightly different from that of Matthew) is in the Didache, 8:2. There are at least ten different versions of the doxology in early manuscripts of Matthew before it seems to have standardised. Jewish prayers at the time had doxological endings. The doxology may have been originally appended to the Lord’s Prayer for use during congregational worship. If so, it could be based on 1 Chronicles 29:11.
Most scholars do not consider it part of the original text of Matthew, and modern translations do not include it, mentioning it only in footnotes. Latin Rite Roman Catholics, as well as some Lutherans, do not use it when reciting the Lord’s Prayer, but it has been included as an independent item, not as part of the Lord’s Prayer, in the 1970 revision of the Mass. It is attached to the version of the Lord’s Prayer used by Eastern Christianity (including Byzantine Rite Eastern Catholic Churches) and by most Protestants. A minority, generally fundamentalists, posit that the doxology was so important that early manuscripts of Matthew neglected it due to its obviousness, (footnote:
It is unclear on what grounds they consider the doxology more important and obvious than, say, “Deliver us from evil.”) though several other quite obvious things are mentioned in the Gospels.
This is
a listing of the variants:
ὅτι σοῦ ἐστιν ἡ βασιλεία καὶ ἡ δύναμις καὶ ἡ δόξα, εἰς τοὺς αἰῶνας. ἀμήν (For thine is the kingdom and the power and the glory, for ever. Amen.) — K, L, W, Δ, Θ, Π, f13, 28, 33, 565, 700, 892, 1009, 1010, 1071, 1079, 1195, 1216, 1230, 1241, 1242, 1365, 1546, 1646, Byz, Lect
ὅτι σοῦ ἐστιν ἡ βασιλεία καὶ ἡ δύναμις καὶ ἡ δόξα, τοῦ πατρὸς καὶ τοῦ υἱοῦ καὶ τοῦ ἁγίου πνεύματος εἰς τοὺς αἰῶνας. ἀμήν (For thine is the kingdom and the power and the glory, of the Father and of the Son and of the Holy Spirit for ever. Amen.) — 157, 225, 418
ὅτι σοῦ ἐστιν ἡ δύναμις εἰς τοὺς αἰῶνας. ἀμήν (For thine is the power for ever. Amen.) — itk
ὅτι σοῦ ἐστιν ἡ δύναμις καὶ ἡ δόξα εἰς τοὺς αἰῶνας. ἀμήν (For thine is the power and the glory, for ever. Amen.) — copsa, copfay
ὅτι σοῦ ἐστιν ἡ βασιλεία καὶ ἡ δόξα εἰς τοὺς αἰῶνας. ἀμήν (For thine is the kingdom and the power for ever. Amen.) — syrc
ὅτι σοῦ ἐστιν ἡ βασιλεία, τοῦ πατρὸς καὶ τοῦ υἱοῦ καὶ τοῦ ἁγίου πνεύματος εἰς τοὺς αἰῶνας. ἀμήν (For thine is the kingdom, of the Father and of the Son and of the Holy Spirit for ever. Amen.) — 1253
omitted by א, B, D, 0170, f1, ℓ 547, it, vg, copbo
Also see here for
a bit more information.