Why wasn't it enough for Paul to just preach that Jesus is the Christ?

Because God had further revelations to give Paul beyond the basic confession that Jesus is the Christ. Proclaiming Jesus as Christ and Son of God was no longer sufficient for the message God intended; Paul had to be prepared through suffering to carry the Gospel of Grace.

Les explains that when Paul went into Damascus and proclaimed in the synagogue that Jesus was the Christ, that was no longer sufficient. It was not enough simply to believe that Jesus was the Christ, the Son of God, because God had further revelations to give the man, so He could not leave him with only that kind of message. Opposition then arose; people were ready to put him to death, and he fled for his life, being let down over the wall in a basket. Les ties this to the governor under Aretas the king keeping the city to apprehend him, showing that suffering and fear began right from the start. It was all part of getting him ready to take the Gospel of Grace.

Why was it not enough for Paul to simply believe and preach that Jesus was the Christ, the Son of God?

It was not enough because God had further revelations to give him. Simply believing and preaching Jesus as the Christ and Son of God was no longer sufficient; the message could not stop there. He had to be prepared, including through suffering and danger, to take the Gospel of Grace.