Commentaries:
After three-and-a-half years and countless miracles, the Jews maintained a high level of disbelief, particularly among the religious leadership. Notice that the source of their doubt was God Himself! Isaiah's prophecy says He had blinded their eyes, and He had hardened their hearts. The reason for this is that if they turned to Him, He would heal them, which may also strike us as odd if we assume that it is always His will to heal.
Perhaps His actions seem unkind or mean-spirited to us, but this passage shows that it was not His will to heal the nation at that time (see also Matthew 13:11-15). The people were still disobeying His commandments and statutes and thus still under a curse. To heal them without repentance would reward their wickedness. God's curse of blindness was so effective that, though Jesus declared them to be blind, they were confident that they saw clearly (John 9:40)! They had no reason even to consider repenting, and therefore, the blindness and other curses continued.
David C. Grabbe
Spiritual Blindness (Part One): The God of This World