This might help. From their own writings:
The Kingdom Interlinear Translation of the Christian Greek Scriptures,
1969 , p. 11: THE DIVINE NAME: One of the remarkable facts, not only about the extant manuscripts of the original Greek text, but of many versions, ancient and modern, is the absence of the divine name... [Comments about divine name in OT]...As the Christian Greek Scriptures were an inspired addition and supplement to the sacred Hebrew Scriptures,
this sudden disappearance from the Greek text seems inconsistent... (Says who?)
Also:
*** it-2 p. 9 Jehovah *** (Insight on the Scriptures book 2)
In
the Christian Greek Scriptures. In view of this evidence it seems most unusual to find that the extant manuscript copies of the original text of the Christian Greek Scriptures do not contain the divine name in its full form.
The Watchtower March 1, 1991 p. 28:
God's Personal Name
At Luke 4:18, according to the New World Translation, Jesus applied to himself a prophecy in Isaiah, saying: "Jehovah's spirit is upon me." (Isaiah 61:1) Many object to the use of the name Jehovah here. It is, however, just one of the more than 200 places where that name appears in the New World Translation of the Christian Greek Scriptures, the so-called New Testament. True, no early surviving Greek manuscript of the "New Testament" contains the personal name of God. But the name was included in the New World Translation for sound reasons, not merely on a whim. And others have followed a similar course. In the German language alone, at least 11 versions use "Jehovah" (or the transliteration of the Hebrew, "Yahweh") in the text of the "New Testament," while four translators add the name in parentheses after "Lord." More than 70 German translations use it in footnotes or commentaries.