Therefore they could not believe,— That is, by the just judgment of God for their obstinate and wilful resistance to the truth, they were so hardened, that the doctrine and miracles of our Lord could make no impression on them, as Isaiah had foretold, Isaiah 6:9-10 where see the note, and also on Matthew 13:14. The meaning therefore is, not that the prophesy of Isaiah was the cause of their unbelief, but that their unbelief was the accomplishment of Isaiah's prophesy. The evangelist, in this quotation, has not confined himself exactly to the words of the prophet, but the sense is plainly the same; and nothing was more usual with the Jewish rabbies, than to quote scripture in this way.
Related Commentaries of John 12:39
John 12:39-40
39 Therefore they could not believe, because that Esaias said again,
40 He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.