Thank you for the question on special revelation, “Why are the revelations of St. Paul not included in the Bible?”
The short answer to the question is that Spirit of God didn’t prompt Him to write those revelations in his letters. The Apostle Paul wrote letters to encourage believers or to answer their questions. It was not Paul’s intention to write a treatise on eschatology.
The Apostle Paul cites revelation that he received from God or heard from the other Apostles. Here are some examples for one’s consideration:
A. Ephesians 1:3–14 are theological revelations regarding God’s eternal plan for the believers in Christ Jesus.
B. Colossians 1:9–20 are theological revelations of the supremacy of Jesus Christ over all creation.
C. I Corinthians 15:12–57 are theological revelations about the resurrection of believers and their transformation.
D. I Thessalonians 4:13–5:11 are theological revelations about coming of Jesus Christ and the believer’s resurrection.
E. II Thessalonians 2:1–12 are theological revelations about the Antichrist.
There are revelations that the Apostle Paul know but didn’t write down. For example, II Corinthians 12:1–4, he says this, “I must go on boasting. Although there is nothing to be gained, I will go on to visions and revelations from the Lord. 2 I know a man in Christ who fourteen years ago was caught up to the third heaven. Whether it was in the body or out of the body I do not know—God knows. 3 And I know that this man—whether in the body or apart from the body I do not know, but God knows— 4 was caught up to paradise and heard inexpressible things, things that no one is permitted to tell” (NIV).
The Apostle Paul states that he heard things and he was not permitted to tell others. That revelation was for him, perhaps to strengthen him in the trials that were to come upon him.
As one reads the Book of Revelation, the content revealed to John is not totally inclusive of all that is happening in heaven and on earth. There are ages to come that God has not revealed beyond the Millennial Period. The Book of Revelation relates to the judgment of God on earth. It does not fully explain the transformation of earth or the Universe.
Can it be that the glory that awaits the children of God is beyond what we think of the rewards in heaven? Paul says this in Romans 8, “I consider that our present sufferings are not worth comparing with the glory that will be revealed in us. 19 For the creation waits in eager expectation for the children of God to be revealed. 20 For the creation was subjected to frustration, not by its own choice, but by the will of the one who subjected it, in hope 21 that[h] the creation itself will be liberated from its bondage to decay and brought into the freedom and glory of the children of God.
22 We know that the whole creation has been groaning as in the pains of childbirth right up to the present time. 23 Not only so, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for our adoption to sonship, the redemption of our bodies. 24 For in this hope we were saved. But hope that is seen is no hope at all. Who hopes for what they already have? 25 But if we hope for what we do not yet have, we wait for it patiently (NIV).
SUMMARY: Followers of Jesus are seeing dimly what heaven is like. We have no conception or perception what it is really like in heaven when we see God in all his glory and majesty.