I would say that the repentance of the people

[Mary Naeem]

Pope Shenouda III

Jonah

I would say that the repentance of the people of Nineveh was mainly due to the readiness of their hearts.

It was this readiness that made God send His prophet to them, and as the Apostle says: "For whom He foreknew... He also predestined" (Rom. 8:29). Indeed readiness of heart plays a major role in the act of repentance.

In the case of the rich young man, the Lord Himself talked to him, and with all the power and efficacy of the Lord's words, he went away sorrowful because the heart within was not ready. This is similar to the stony places which do not yield fruit no matter how good the seeds are and no matter how experienced the sewer is,
The heart of the young St. Anthony, however, was ready to hear the word of God. When he heard in the church the same verse said to the rich young man, he was deeply affected and carried it out wholeheartedly. Likewise were the hearts of the Ninevites.