Why? Because even in the midst of their torment I do not believe that they will be able to repent. They will endure the torments of hell saying, like the spoiled children they are, “But this is so unfair!! I didn’t do anything wrong!”
This must be so because this is the nature of the unforgivable sin. The unforgivable sin is unforgivable not because God cannot forgive it, but because the sinner refuses to be forgiven. They refuse to be forgiven because they refuse even to the end of eternity to admit that they did anything wrong.
In other words, hell must be eternal because their lack of repentance is eternal. They are on a path that they simply cannot and will not deviate from. Their denial of guilt and self justification has become not just a habit, but an addiction. Chained in their self righteousness there can be no escape.
The terrible thing to contemplate is that I hope to believe that God’s mercy even then would extend to them. I would like to believe that God’s mercy would overcome all and that he would forever be reaching down into the pits of hell to rescue the perishing. All they would need to do is look up just once and ask for that mercy.
But they will be unable to. They will rebel in their pride and resist forever.
That’s why they are in hell, and that’s why it will be eternal.