John says people were not able to believe because God had blinded their eyes and hardened their hearts, as revealed by Isaiah. How does God go about blinding and hardening? The clue is in the next verse:
Isaiah said this because he saw Jesus' glory and spoke about him (v. 41). More literally, Isaiah said "these things," that is, both quotes from Isaiah are in view. "Isaiah could report on Christ's saying concerning the predestined unbelief of the Jews because he had in his vision [in Isaiah 6] seen the glory of the crucified Son of God" (Dahl 1976:108). Isaiah
spoke about him, and thus the verbs that have God as their subject in Isaiah are taken here as referring to Jesus (cf. Carson 1991:450). This would be in keeping with John's earlier statement that no one has ever seen God (1:18), but we have beheld the glory of the only Son (1:14) so that those who have seen the Son have seen the Father (14:9). For the glory of God revealed in Jesus is the self-sacrificing love evident in the Suffering Servant. The scandal of the arm of the Lord revealed in the Suffering Servant corresponds to the scandal of the love of God revealed in Jesus. And as the revelation of the arm of the Lord produced mute disbelief in Isaiah 52:12--53:1, so the glory of the Lord revealed in Jesus has produced disbelief. God's revelation of his glory has caused the blindness and the hardness (cf. Jn 9:39-41). The same sun that melts wax, hardens clay (Origen
On First Principles 3.1.11). The hardness of heart found in these opponents is that which rejects God's offer of mercy. Specifically, it is his offer of healing that they reject. This offer of healing, which has blinded and hardened, has come from God through Christ.
John 12 Commentary - John Reflects on the Tragedy of Unbelief - BibleGateway.com