You may like the insight my pastor offered for this one.
The theory goes as follows (in my own words).
Satan saw that certain angels were tempted by the beautiful women found in Genesis 6. Being the shrewd corrupt lawyer he is, he figured that if they could procreate with them, that this would bring about a mutant race, and from then on the promised seed of the Messiah could not come... For the Messiah had to be a man, as God created man to be.
On hearing this, the angels accepted this temptation because they strongly desired to see Satan reconcilled back into the fold. We must realize that one third of the angels preferred Lucifer over the Lord. Lucifer was greatly loved by the angels. The angels that stayed with the Lord were somewhat torn in the beginning, because they could not understand fully God's reasoning (God is omniscient) as to why their beloved Lucifer was to be rejected forever.
So, these particular angels thought they could have their cake and eat it too! They can give in to taking these beautiful women they lusted for, and win back Satan into the family! They rebelled, and were soon afterwards to be thrown into prison.
Now, these spirits in prison did not learn the outcome after the flood. They had been locked up for their rebellion. They had a burning hope that kept them persevering, that they had succeeded, and that the human race had become mutant. For that reason, the Messiah could not come. And, if the Messiah could not come, God's Word is not perfect! If God's Word is not perfect! Then his judgement also is not, and all those due for punishment would have to be made free!
When Jesus appeared to them and preached to them, he was showing them that their hero, Satan, had failed in his plan for gaining his freedom. There is to be no false hope given in Hell. These angels were a unique group. They needed to have all hope removed. For, the greatest pain of Hell will be the absence of hope. One can be tormented as long as they believe they have hope.
Grace and peace, GeneZ