Although he had performed so many signs in their presence, they did not believe in him.
This was to fulfill the word spoken by the prophet Isaiah: "Lord, who has believed our message, and to whom has the arm of the Lord been revealed?"
And so they could not believe, because Isaiah also said,
"He has blinded their eyes and hardened their heart, so that they might not look with their eyes, and understand with their heart and turn— and I would heal them."
Isaiah said this because he saw his glory and spoke about him.
References for John 12:41
Nevertheless many, even of the authorities, believed in him. But because of the Pharisees they did not confess it, for fear that they would be put out of the synagogue;
for they loved human glory more than the glory that comes from God.
Then Jesus cried aloud: "Whoever believes in me believes not in me but in him who sent me.
And whoever sees me sees him who sent me.
I have come as light into the world, so that everyone who believes in me should not remain in the darkness.
I do not judge anyone who hears my words and does not keep them, for I came not to judge the world, but to save the world.
The one who rejects me and does not receive my word has a judge; on the last day the word that I have spoken will serve as judge,
for I have not spoken on my own, but the Father who sent me has himself given me a commandment about what to say and what to speak.
And I know that his commandment is eternal life. What I speak, therefore, I speak just as the Father has told me."