@Background Pony #B53D  
Chrysalis almost accepted and was never offered it again while Starlight rejected Twilight multiple times before giving up. Cozy was literally never offered or given a second chance. Discord was given one despite him clearly not wanting it.
 
You can’t force someone to change, but you can help them to it, which sadly the trio never received.

@Background Pony #A33F
 
Except for Tirek (and only because we weren’t shown that) all of them actively refused any potential second chances given to them:
 
 

 
As they say, you can force a man into the baptismal waters but you can’t force him into believing.

@Background Pony #DAC7  
Except for the fact that that Discord used to be just as bad as they were, only that he did it for fun and Starlight’s excuse went out the window when she tried to kill three of her villagers and go for petty revenge. Also, let’s not forget the methods Discord used to keep the trio in-line and ready to lose again under his whim, as @Mediocre-Grimm pointed out. That, and his actions in the past was more then some big oopsie.
 
The fact of the matter is that those three never got a second chance despite deserving them just as much as any other reformed villain.

Many armchair psychologists are trying to put a name to their ruffled feathers at something they can only see the surface details with.  
Discord made a (big) mistake.  
Starlight wanted everyone to be on equal terms.  
These three wanted to murder and rule everyone and never actually attempted to be friends with anyone, not even amongst themselves.  
It’s not hard to notice unless you have a petty vendetta.

@Background Pony #669E  
But even that is marred by the lack of attempts to rehabilitate them an Discord’s constant relapses and role in the whole debacle as @Mediocre-Grimm pointed it out. As it is there is no proof that these three are any more irredeemable then of Discord or Starlight were.

@Background Pony #12E5  
I know a lot of people felt like that, but personally. I disagree. The point, I think, is that while anyone is capable of becoming a better person, there are some people who will never want to, and such evil cannot be allowed to go free.

@Background Pony #A33F  
Because the writers were done doing anything logical for the show

@Mediocre-Grimm  
It’s reasons like this that I firmly believe that the trio didn’t deserve neither stone nor Tartarus. Why shouldn’t they deserve the same kind of help that Discord got?

@Mediocre-Grimm  
He represents the following. (Another threads comment)  

@Mediocre-Grimm  
….well crap

It’s interesting how Discord managed to embody all three of these vices at once in season nine, and yet he walked free.
 
He deceived the Legion of Doom and the Mane Six.
 
He manipulated the Legion of Doom into becoming obstacles for the Mane Six.
 
He betrayed the trust of his friends and had things gone to plan, would’ve betrayed and sacrificed the Legion of Doom to feed Twilight’s ego.

This is … scarily close to how recorded history actually work.

@Background Pony #A33F  
I have to agree.

@Background Pony #DAC7  
Pretty sure he does considering what he did.
 
@Background Pony #12E5  
Indeed. What makes it worse for me is that I just can’t enjoy otherwise genuenly good content like this because those thoughts of the trio’s fate keep popping up.

@Background Pony #12E5  
Eyup :(

Years later, and this still upsets me. This was so anthetical to the show’s premise it actually led to me being unable to watch any of the episodes. It didn’t help that they made the trio protagonists in earlier episodes, and then in the finale acted like none of the character development ever happened. Sorry for the rant, this is a nice strip. But it’s depressing, since it shows Twilight would rather use them for an example instead of helping them.

@Background Pony #A33F  
Not really.

@Background Pony #44E6  
Truth. If they deserve to be there then so does Discord.

@Background Pony #DAC7  
Nah. Thank ye though.

@Background Pony #44E6  
You should get over it.

I like this version of anon!

There is one statue missing alongside them. The vice of hypocrisy discord.  
ps. Anons like Pokerface unimpressed champ.

I do like the idea of this immortal being popping on this universe now and again and going “Oh. Huh. That’s what happened to them.” And the symbolism for the villain statue is fantastic.