"Now the poor man died and was carried away by the angels to Abraham’s bosom; and the rich man also died and was buried. In Hades he lifted up his eyes, being in torment, and saw Abraham far away and Lazarus in his bosom. And he cried out and said, ‘Father Abraham, have mercy on me, and send Lazarus so that he may dip the tip of his finger in water and cool off my tongue, for I am in agony in this flame.’ But Abraham said, ‘Child, remember that during your life you received your good things, and likewise Lazarus bad things; but now he is being comforted here, and you are in agony. And besides all this, between us and you there is a great chasm fixed, so that those who wish to come over from here to you will not be able, and that none may cross over from there to us." (Luke 16:22-26)
It is clear from the cited passage above that there are no additional chances for salvation when the souls of men enter the direct presence of God. In other words, we all have an upcoming appointment with Deity. The day of judgement is inescapable. Thus, those who have not trusted in Jesus Christ as Lord and Savior in this life will be eternally condemned. Hebrews 9:27 and Revelation 20:11-15 also reflect these biblical truths.
What needs to be understood is that God does not have to save any of us from our sins. He offers to us eternal life because He is loving and merciful. The better question to ask is not why man cannot be saved after death, but why God ought to save anyone at all? This is simply the way that God has ordained things to be. He tells us that we will not have another opportunity to repent after death. So right now is the best time to repent of sin. Today is the day of redemption (2 Corinthians 6:2).