Allah did not forgive Iblees simply because he did not repent. We know the story of Adam(alayhi salam) and Iblees. When Adam realized and acknowledged that he has fallen to sin, he immediately realized his mistake and repented to Allah and asked Allah to forgive him. And Allah did.
They said, "Our Lord, we have wronged ourselves, and if You do not
forgive us and have mercy upon us, we will surely be among the
losers." [Surat Al-'A`rāf 7:23]
And Allah accepts his repentence.
Then Adam received from his Lord [some] words, and He accepted his
repentance. Indeed, it is He who is the Accepting of repentance, the
Merciful. [Surat Al-Baqarah 2:37]
This applies to us as well, every human being falls into sins but the best of them is those who repent to Allah and asks for his forgiveness and Allah is ready to forgive them.
As for Iblees, I can list his transgressions.
First, he refused to obey Allah's command to bow to Adam (alayhi salam).
And We have certainly created you, [O Mankind], and given you [human]
form. Then We said to the angels, "Prostrate to Adam"; so they
prostrated, except for Iblees. He was not of those who prostrated.
[Surat Al-'A`rāf 7:11]
Secondly, Satan said it is since he is better than Adam (Alayhi salam). Satan did not acknowledge his sin and was arrogant of his creation i.e. by saying he was better than Adam (Alayhi salam). It is not permissible to be arrogant and boastful of ones creation. One must humble himself before Allah.
[ Allah ] said, "What prevented you from prostrating when I commanded
you?" [Satan] said, "I am better than him. You created me from fire
and created him from clay. [Surat Al-'A`rāf 7:12]
But Satan was wrong. Fire is not better than clay. This is because clay causes growth. Plants germinate from soil and it leads to growth. However, fire is only destruction. So, clay is better than fire and Satan was wrong.
Thirdly, Satan tried to blame Allah and the divine pre-decree for his sin and mistake.
[Satan] said, "Because You have put me in error, I will surely sit in
wait for them on Your straight path. [Surat Al-'A`rāf 7:16]
So Satan blames Allah of putting him in error and leading him astray. It is not permissible for one to blame the pre-decree for his sins and only when some calamity befalls him, he accepts it as pre-decree and remains patient. So, whatever sin we do is the doing of our own hands and we cannot blame it on pre-decree as Allah says (interpretation of meaning):
What comes to you of good is from Allah , but what comes to you of
evil, [O man], is from yourself. And We have sent you, [O Muhammad],
to the people as a messenger, and sufficient is Allah as Witness.
[Surat An-Nisā' 4:79]
Since, Satan never repented and was arrogant, boastful and disputed with Allah, there is no question of forgiving. Mercy is for the one who repents.
Source: Explanation of Story of Adam and Iblees by Shaykh Salih Al Fawzaan