"The Zone" is the result of Ani's attitude, conviction, resolve, and tactical fortitude being greatly sharpened into a razor point by his emotions - combined with the effects of Force rage that greatly enhance both his physical and Force stats.
Whereas pre-cyborg Vader has Force rage, but he's also a dull, blind, lunatic.
The zone was never realized in Vader because he just wasn't feeling it, he had a bad attitude - he could have regenerated his lungs (Shadows of the Empire) if he could have maintained sufficient rage during his time the meditation chamber using a form of Force healing. Even as a cyborg, if he'd had that attitude, and been lifted from despair he could have been a real physical threat to Sidious (Rise of Darth Vader) but still wouldn't try it because Sidious was the closest thing he had to a friend (Rise and Fall of Darth Vader).
Mortis is different, his powers are being amplified to an unknown extent by the mystical effects of this platform he's on, which is the prime vocal point of the Force itself.
This is like when Cyborg Vader was on Mimban, while holding the Kaiburr Crystal his powers were a thousandfold (RaFoDV) - WAAY more power than the zone - it also made him blood-lusted and able to use kinetite to achieve Force lightning without it passing through his cybernetics (Splinter of the Mind's Eye).
Zone wasn't even his peak - his natural plateau would be similar to FoTJ Luke, and in Empire of Dreams documentary on ANH, Lucas states it's twice that of Sidious' from ROTS-ROTJ - plus whatever being in the zone mindset would do for him plus whatever achieving living union with the Dark or Light side would do to him - which amped Luke dozens of times over in one of his encounters with Abeloth.