Because they repented that generation of Ninevites was not destroyed. That's the sign of Jonah. Jesus was the same sign. If that wicked generation did not repent, it would be destroyed. They didn't and were. In A.D. 70, 40 years later, Rome sent its armies and destroyed everything beginning in Galilee to Judea even burning and dismantling their religious temple. They got their sign alright. Sometimes getting a miraculous sign from heaven is not good.