Why were the Jewish leaders surprised by Peter and Johns' preaching?

Answer 1

They were surprised because Peter and John were just ordinary men (fishermen) not educated but the talked with authority and knowledge.

Act 4:13 The members of the Council were amazed to see how bold Peter and John were and to learn that they were ordinary men of no education. They realized then that they had been companions of Jesus.

Answer 2

When the apostles Peter and John stood before the Sanhedrin, the judges of that court considered the men before them to be as what is stated in Acts 4:13. Oh, they could read and write, but they had not received any rabbinic training in study of the Scriptures, meaning that their education was elementary and not from the schools of higher learning.

Even so, Peter and John along with the other disciples had proved to be effective teachers, moving many of those who listened to them to become believers. How did these ordinary men become such extraordinary teachers? After listening to Peter and John, the court "began to recognize about them that they used to be with Jesus." (Acts 4:1-4) True, they had received holy spirit. (Acts 1:8) But it was also obvious that Jesus had trained those men.