John 12:39
The reason why they were unable to believe is given by Isaiah elsewhere, in these words--
John 5:44
How can you believe in me, when you receive honor from one another and do not desire the honor which comes from the only God?
John 6:44
"No one can come to me, unless the Father who sent me draws him to me; and I will raise him up at the Last Day.
John 10:38
If I am doing it, even though you do not believe me, believe what that work shows; so that you may understand, and understand more and more clearly, that the Father is in union with me, and I with the Father."
2 Peter 2:14
They have eyes only for adulteresses, eyes never tired of sin; they entice weak souls; their minds are trained to covet; they live under a curse.