Here’s why fallen angels were not forgiven and how their sin differed from mankind’s.
Pure intellect

Humans feel hunger pangs, fatigue, irrational mood swings, sadness, anger, fear, etc. All these factors interfere with our decision-making ability. Angels aren’t physical beings; they are spirits.
They don’t experience biological drives or irrational mood swings. The angels made their decision based purely on will and desire, making their sinful choice that much more evil.
Eternal state

Humans live in linear time, which allows us the space to grow, learn, and pivot. Angels have always been in a unique state with regard to time.
Many believe that for a being who exists outside of time, a single choice is a fixed state. Once they turned their will away from God, it’s who they literally are.
Direct knowledge

Angels exist in the direct presence of God. Instead of living by faith and intuition, they had intellectual knowledge. They didn’t have to guess at what was true; they simply saw it.
When someone is presented with absolute information and still chooses to deny it, there can be no new revelation that will change their mind.
No tempter

When humanity fell from grace, we were tempted by someone else’s lie. That leaves room for forgiveness because we were the victims of deception.
The angels were tempted by no one. Pride and rebellion started within themselves. They are wholly responsible for every starting point of their downfall.
Rejected glory

Imagine walking away from a small candle versus walking away from the sun. Angels saw God Face-to-face.
To look into the perfect Goodness of God and decide you hate Him is the moral equivalent of hardening your heart 100%. If serving the source of all joy wasn’t worthwhile, then nothing is.
No mediator

In order for God to rescue a species, He must take on their nature so that the gap between Him and them can be bridged. Christianity teaches that God took on human nature so that humans could be rejoined with God.
But angels? God did not become an angel to suffer for their transgression. Without a common link between the Divine and the angelic nature, there is no legal or spiritual path back.
Static will

Humans are dynamic. We are growing and developing every day of our lives. Angels are simple creatures.
This means they are not composed of parts that get better or worse over time. Their decision to disobey defines their character.
Total independence

Humans are born into a broken world. We aren’t born programmed to know right from wrong. Our families and peers influence us for better or worse.
Angels were created as fully adult spirits. They each chose on their own. Fallen angels can’t blame their childhood or their friends. Their decision was totally autonomous.
High authority

It’s a political fact: the more power you have, the more accountable you become for your actions. Angels have greater responsibility than humans because they are stronger and smarter than we are.
They knew better, so their fall was objectively worse than ours. They abused greater power, so their sin was a greater crime.
Zero remorse

Forgiveness requires a person to actually want to change. Fallen angels want nothing to do with repentance.
They are so proud that they would rather rule a dark void than serve in heaven. You cannot forcefully forgive someone who remains in an active state of war against you.
Moral order

If the angels could just disobey God and not suffer eternal consequences, then free will is worthless. We are told that we will suffer eternal consequences for our sins.
In order for this not to be a meaningless threat, someone had to show that you will be held accountable for your rebellion. The angels are that example.
Sources: Please see here for a complete listing of all sources that were consulted in the preparation of this article.