Salam everyone
I believe the ability to obey or disobey God comes with free will. Those created and endowed with free will ( humans, jinns etc) technically always had the option/ability to disobey God. Knowing this God created creations with free will.
We do not know whether this was the first time Iblis chose to disobey God. But maybe it was the one time Iblis chose to disobey God concerning us, humans and since this is about us we are informed about it. Maybe Iblis and other creatures or other jinns did disobey God even before this incident. We don't know. Also, at the end of the day we will be punished for disobeying God, we won't be forgiven just because satan vowed to mislead us. We will be punished for using our free will to disobey. This I believe proves that it all comes down to free will and how we excersise it. Our free will and ability to obey or disobey is the test designed for us. Satan is a powerful enemy but we can defeat him if we excersise our free will and just obey God. Iblis will not be able to mislead true believers.
The incident mentioned in Quran there Iblis refused to follow orders because of his pride and his unfounded assertion that he was better than humans. He was asked to give justification for his actions and a chance to repent but He refused. Not only that He vowed to mislead us. All this has a purpose but we don't know what!
I believe he disobeyed because he had the ability to disobey and his justification for doing so was his pride. He keeps on doing it also because he wants to prove that he was right about us, that we are a bunch of ungrateful and transgressing creatures. This he does out of pride and his need to be correct and prove his point to God. He is able to do all this because he is granted respite. Why is he granted respite? The exact reason is only known to God. But this can in the end help us find out who among us , creatures with free will, is the most noble and nearest to God.
God loves those who repent, Iblis did not repent, as far as we know, far from repenting, he is hellbent on proving his point out of sheer arrogance. This is why he does what he does. Moreover, his fate as the one fallen from grace has been sealed, he has nothing to lose but a lot of things to prove ( atleast according to him). A normal humble creature with free will would have repented and asked for forgiveness but satan is haughty and arrogant. Plus, there is a window of opportunity or a specific amount of time till which u can repent. Even for humans, we can ask for forgivenss till a specific time, before our death and before our hearts are sealed and our fates are sealed. Maybe satan has run out of his God decided amount of time allotted to him to repent. Maybe Iblis' heart has been sealed and his fate has been decided. Just the way some of our hearts are sealed and we are not guided. These decisions are taken by God out of His infinite wisdom. He knows our souls better than us. So Iblis will keep on misleading till the day He meets his punishment like the rest of us. The day all creatures with free-will will get to know his/ her position in God's heart and His kingdom.