Why do people not respond to the gospel?

Paul said "if our gospel is veiled, it is veiled to those who are perishing" (2 Corinthians 4:3). May I say the problem was not with the message or the messenger, but with the hearers headed for hell. The preacher cannot persuade people to believe; only God can do that.
Paul also says Satan has blinded the minds of the unbeliever (verse 4). Satan blinds men to the truth of God's Word through the world system he has created. The majority of people have the mind-set that is expressed by this world system. Without a godly influence, man left to himself will follow that system, which panders to the depravity of humankind and deepens their moral darkness.
"For we do not preach ourselves, but Christ Jesus" (verse 5). False teachers accused Paul of preaching for his own benefit, yet they were the ones guilty of doing so. Paul never promoted himself, but always preached Christ Jesus as Lord. Notice verse 6, "For it is the God who commanded light to shine out of darkness, who has shone in our hearts to give the light of the knowledge of the glory of God". The same God who created physical light in the universe must create supernatural light in the soul of the unbeliever before that person can respond to the gospel. Child of God we are to be faithful, to proclaim the gospel of Christ, and leave the results to God. You think about that.