that's because Mortis Anakin doesn't equal zone Anakin....
imo it surpasses it .....Mortis Anakin... is completely different.
we saw him command light and dark...we also saw little control of light and dark...
so again...from what I have read of zone Annie doesn't convence me that they equal...
when Mortis Anakin was clearly more powerful but was still easily swayed to the dark side.
after seeing the future,he still chose the darkside.....
imo both versions were easily swayed to the darkside....
As for the Force Nexus any Jedi or Sith can benefit from this massive amp...
we saw his zone take down Dooku/but ended up limbless against kenobi .....
we saw his Mortis version amplified (somewhat) (still debatable)submitting the light and dark but not so much as controlling
them....he on the other hand was seduced and controlled by The Son....