The true cause that does not let man repent and look at his true spiritual condition is pride.
Pride does not allow man to repent despite knowing that he is sick of death, it is the pride that every human being prefers to please and obey rather than give in, because pride puffs up their hearts and separates them totally from God.
Sometimes when we look at the actions of some people we think that man does not have enough knowledge about his condition of sin, but we are mistaken then, man according to Romans 1, has so much knowledge about God that they hate him, and it seems that the true Problem is moral and not intellectual. Let's see what Romans 1: 18-21 tells us:
18 For the wrath of God is revealed from heaven against all ungodliness and injustice of men who unfairly stop the truth;
19 because what is known of God is manifest to them, for God manifested it.
20 For the invisible things of him, his eternal power and deity, become clearly visible from the creation of the world, being understood through the things done, so that they have no excuse.
21 For having known God, they did not glorify him as God, nor did they give him thanks, but they became puffed up in their reasoning, and their foolish heart was blackened.
In this way we realize that man has a conscience that indicates when he is doing wrong and for this reason he will be judged if he never gets to hear the gospel and know God.
Pride cannot be stronger than the glory of God, for God has done everything so that man can be forgiven and reach his mercy.
So there is no excuse before God to acquire his eternal mercy and achieve his grace.