Often it is difficult to provide answers to "why" questions because unless the reasons have been documented or a person is alive to attest to why something was done, we do not really know. All we can do is make assumptions.
THE INTERNATIONAL BIBLE SOCIETY provides the following information:
Some wonder why just these 66 booklets were chosen. Why not 65 or 67? Why was the sometimes puzzling booklet of Jude included to the exclusion of other edifying scriptures? To these questions we reply that these books are the ones that God himself has chosen to preserve for us, and he has not told us exactly why. Together they form an immeasurable treasure, and in them we find God's matchless gift to his people. Here we are moved simply to trust in his providence as he led his people through the years and gave us the most honored and powerful and comforting volume in the history of humanity, the book known as the Bible.
The book of Revelations follows the apocalyptic tradition of Ezekiel and Daniel by using Old Testament figures of speech and symbolism (sun, moon, stars, beasts, horns).
What is interesting about the Bible is what the first sentence in Genesis 1:1 and the last sentence in Revelation 22:21 state:
In the beginning God created the Heavens and the Earth...The grace of the Lord Jesus be with all the saints. Amen
Essentially the Bible is about the Lord God creating the Universe so that Lord Jesus Christ could be with all the saints. The book of Revelation is very much about how Lord Jesus Christ accomplishes this. Although written symbolically, the message that the book of Revelation contains is very clear:
The revelation of Jesus Christ, which God gave him to show to his servants what must soon take place; and he made it known by sending his angel to his servant John, who bore witness to the word of God and to the testimony of Jesus Christ, even to all that he saw....I warn every one who hears the words of the prophecy of this book: if any one adds to them, God will add to him the plagues described in this book, and if any one takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book. He who testifies to these things says, “Surely I am coming soon.” Amen. Come, Lord Jesus! (Revelation 1:1-2; 22:18-20).
Because of the clarity of the message, one has to assume that this would be one of the main reasons why people accepted the Revelation to be a genuine communique from Lord Jesus Christ to the prophet John. Besides this, the book's authenticity is summed up by the Christian Courier:
Irenaeus (A.D. 180), a student of Polycarp (who was a disciple of the apostle John), wrote that the apocalyptic vision “was seen not very long ago, almost in our own generation, at the close of the reign of Domitian” (Against Heresies 30). The testimony of Irenaeus, not far removed from the apostolic age, is first rate. He places the book near the end of Domitian’s reign, and that ruler died in A.D. 96. Irenaeus seems to be unaware of any other view for the date of the book of Revelation.
As for being Gnostic, the book of Revelation exalts Lord Jesus Christ as the Creator along with our Heavenly Father. Gnosticism denies this. Therefore, the book of Revelation is not of Gnostic origin.
We can say that the book of Revelation is not Gnostic. It appears to have been included in the Christian canon because Irenaeus attested to its authenticity. Its inherent witness attests to its explanation of what will happen in the Church age from Pentecost to the creating of a new Heaven and a new Earth. Key features of its imagery is taken from the book of Daniel, of which the book of Revelation provides further insight that is backed up by the Gospels and the writings of the Apostles in the Epistles.