Why Was Jonah afraid to go nineveh?

Because of their sins:

Jonah 1:1-2New King James Version (NKJV)

1 Now the Word of the Lord came to Jonah the son of Amittai, saying, 2 "Arise, go to Nineveh, that great city, and cry out against it; for their wickedness has come up before Me."

God forgave the city because they fully repented - unlike the Children of Israel who continuously fell backwards:

Jonah 3:10New King James Version (NKJV)

10 Then God saw their works, that they turned from their evil way; and God relented from the disaster that He had said He would bring upon them, and He did not do it.