Because of Their Unbelief

These Nazarenes knew precisely who Jesus was and they were likely remembering the scandalous rumors that had no doubt circled around a young virgin girl named Mary who had become mysteriously pregnant around 30 or so years earlier and "this man" was "her" son. Their questions presuppose that the wisdom and power of Jesus did not come from his unique identity. They did not recognize him as from God and certainly didn't recognize the displays of his divinity as the Son of God. His own disciples were just starting to connect those dots.
These Nazarenes didn't even recognize him as the great teacher and prophet that he was. To them, he was just ordinary. A carpenter, just one of them and the end of verse 3 says, And they took offense at him. Friends this was the day that Nazareth committed spiritual suicide.