FF6 is edgy enough. Can you think of any other computer games where the antagonist succeeded in all his goals and the world is still doomed at the end?
Kefka wanted to rule the world. He did.
Kefka wanted to destroy the world and watch it decay into nothingness. He did.
Kefka wanted a cult of nihilists who did nothing but adore him. He got it.
Kefka wanted to become God. He did.
Kefka wanted to die and the death of God/himself to be the crowning glory to his "monument to non existence". He did.
FF6 is far, far more edgy than FF7 or any other RPG to date in that respect. It wholly subverted the narrative structure of hero stories in games in a way that hasn't ever been replicated since. It can't even really be classified as a tragedy because there is no after affect after the tragedy takes place, there's just oblivion and inevitability. FF7 took on a "life and rebirth goes on" to take the edge off, FF13 likewise saw the destruction of God and the universe giving birth to a new one.
This didn't happen in FF6. God was an asshole, you killed him, and the world is doomed without magic and with most of the population already dead it's not going to get any better.
It's likely for that reason it wasn't as popular, and thus hasn't been copied in that way since.