So Jonah arose, and went to Nineveh, according to the word of Yahweh. Now Nineveh was an exceedingly great city, three days' journey across.
Now, with regard to the "chief reason," Jeremiah and Isaiah both were doubtless influenced by Jonah, especially Isaiah who, in full harmony with the inevitable deductions that appear mandatory in the Book of Jonah, prophesied again and again the rejection of Israel and the acceptance of the Gentiles into the kingdom of God.  Romans 9-11 is fully devoted to this.