The enemies of Jesus by demanding a sign, they only wanted to challenge. They thought that Jesus would do something to prove Himself right or wrong.

Jesus did not perform a miracle for them simply because He knew that they had seen Him several times healing the deaf, restoring sight to the blind, multplying bread, rising the dead, but still they did not believe. They kept on looking for more from Him, therefore Jesus knew that they would never believe. Still Jesus knew that they were looking for any slight mistake from Him, so that they could find reasons enough to convict Him before the crowds that followed Him whenever He went.