Well, in the FASA setting, they simply vanished for no readily apparent reason around the time of the Wrath of Khan (it gets a mention in the movie sourcebook updates).
As for canon Trek, it's never explained. It's possible that they decided the Khitomer Accords fulfilled their prediction so they didn't need to enforce things anymore.
Alternatively, their power was never as great as they claimed and they could only control territory close to Organia.
I'd guess you could come up with any solution you wanted!
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)