Something to do with how by falling they have permanently changed themselves. Almost into a negative version. I think it was in a recent WOJ how that their grace or something converts into a negative. It makes your argument stronger in some ways, because perhaps they cannot change because they never did (being mostly outside time). But that raises questions. Why are the Fallen given no option at all the redeem themselves? It's a central tenet of many religions. What does that say about the nature of TWG or the universe? Or is it more the fact that they are no longer really the being they were before and so cannot be redeemed. They are something else.
I don't know why it would be that way other than the real world explanation I heard of them existing outside of time or TWG being unwilling or unable to forgive. I've always found fallen angel type characters that can't be redeemed sad.
Uriel seems to suggest that the choice is still somewhat an option for him...the the consequences of course could be catastrophic.
Or it's Michael's choice that would cause Uriel to fall, not Uriel's?
Nicodemus seemed to believe Uriel could still choose to smite him, to judge him.
Or Nicodemus was just taunting Uriel, believing (incorrectly) that Uriel could do nothing, but it turned out Uriel was stalling to give Butters time to get the sword?
Then again...perhaps if they chose to betray (somehow) the other Gods and that seems like it would have been before that...but who knows?
Or that choice happened at the same "time?"
I ended those sentences with question marks because, with the information we have, it's pretty close to a coin toss for me.
What if the Angels exist outside of time but can't see past the fork in the universe where a "Choice" is made?
I'm not sure how that would work, but I'm not sure how anything "outside of time" would work.