At the preaching of Jonah, Nineveh was very fast to repent - quite probably the citizens were aware of their sins as sins but simply had no impetus to change their ways, and no God to change them for. Jonah's message must have spread through the town like fire, one person to the next ahead of his ability to proclaim it to them; even the king was told, and lost no time in donning sackcloth.
Contrast that to the Israelites and their leaders – they were so convinced of the rightness of their ways that they would not listen. It was not God's prophet but God Himself who spoke to them, and they still ignored Him. “An evil generation seeks a sign,” and an evil generation will receive a sign.