Well that and a lot of those purged who had been genuinely betrayed, had been loyal MLs who fit the "Stalinist" label right up to their execution. Even for those who did oppose Stalin they can hardly give actual focus to, they cannot say anything more about the likes of Kamenev or Zinoviev since they are "authoritarians." They can try to offhandedly say "look how Stalin treated even his allies", but would never justify men who truly agreed with his line like Krestinsky. Hell it is still kinda surprising that Bukharin is loved by guys like Gorbachev and somewhat rehabilitated in the west given he was an attack dog against Zinoviev.