Why did Paul not tell others what he saw and heard in heaven?

Paul refrained because God told him not to share what he saw and heard in paradise. The words were unspeakable and not allowed for a man to utter. He also forbore lest any man should think of him above what they saw or heard.

Les explains that Paul had an experience of being caught up into paradise, the heaven of the heavens. There he heard unspeakable words and angelic conversations and saw the very throne room of heaven. It was so thrilling that it was all he could do to keep from bragging. Though he was human and wanted to share what he had seen and heard, he knew he dare not because God told him not to. God also had to do something physically to him so he would never forget he was not to share it. In verse 6 Paul says he forbears, lest any man should think of him above that which he sees or hears of him.

2 Corinthians 12:6

For though I would desire to glory, I shall not be a fool; for I will say the truth: but now I forbear, lest any man should think of me above that which he seeth me to be, or that he heareth of me.

What was the danger if Paul revealed his revelations of glory?

The danger was that people would exalt Paul above measure and think of him more highly than they should. The world would beat a path to his door because he had been to glory and returned, so God forbade him to share it.