Don't forget that the Book of Revelation is, Revelation 1:1 (WEB):
(1) This is the Revelation of Jesus Christ, which God gave him to show to his servants the things which must happen soon, which he sent and made known by his angel to his servant, John,
It is Jesus revealing new things concerning the future, so it's not that surprising that it's not also revealed in other older books of the Bible.
Revelation 20:6 (WEB):
(6) Blessed and holy is he who has part in the first resurrection. Over these, the second death has no power, but they will be priests of God and of Christ, and will reign with him one thousand years.
Only those, and everyone of those, that have a part in the first resurrection are blessed with having eternal life (and immortality). The rest of mankind have a different resurrection and path to eternal life. The second, and last, resurrection is described in Revelation 20:13 (WEB):
(13) The sea gave up the dead who were in it. Death and Hades gave up the dead who were in them. They were judged, each one according to his works.
Concerning verse 5, I have found the following comments:
Question:
Kindly explain Rev. 20:5 "But the rest of the dead lived not again until the thousand years were finished."
Answer:
Scholars are agreed that this text is an interpolation. We must remember, however, that it is one thing to be legally, or officially dead, and another thing to be actually dead. But as Jesus said to some, He recognized as alive only those who accepted Him. Those who had not the Son had not life in any sense or degree; those who have the Son, have the beginning of life reckoned to them. The world, however, during the thousand years will have the opportunity, not only of being awakened, but of having fulness of life. If, therefore, after they are awakened, they will go on and render obedience to the laws of the Kingdom, they will be lifted up, up, up out of death to perfection and life.
In Rev. 20:5, the words "But the rest of the dead lived not again until the thousand years were finished" are spurious. They are not found in the oldest and most reliable Greek MSS., the Sinaitic, Vatican Nos. 1209 and 1160, nor in the Syriac MS. We must remember that a few passages found in the modern copies are additions which do not properly belong to the Bible. Since commanded not to add to the Word of God, it is our duty to repudiate such additions as soon as their spurious character is established. The words indicated probably crept into the text by accident, in the fifth century; for no MS. of earlier date (either Greek or Syriac) contains this clause. It was probably at first merely a marginal comment made by a reader, expressive of his thought upon the text, and copied into the body of the text by some subsequent transcriber who failed to distinguish between the text and the comment.