NOTE: I have not yet read Bionicle book 3 "Escape from the Underworld" yet, so I may be missing some information that would contradict my theory.
Here's a theory of mine. I think Kulta was either more powerful than Umarak, or he was more trusted by Makuta.
Kulta was sent to find and destroy the mask of creation, the mask belonging to the one person strong enough to stop Makuta.
Umarak was sent to find the mask of control, only for Makuta to betray Umarak and force him into wearing it, which caused him to mutate and probably has turned him into Makuta's mindless slave.
Given that the mask of creation is likely much more powerful than the mask of control, it's surprising that Kulta could wear the mask of creation, and not be affected so drastically unlike Umarak. Not to mention, Kulta is in charge of an army that are all controlled through his power.
Basically, I believe Kulta was not only more powerful than Umarak, he was likely far more trusted by Makuta, since he gave Kulta such an important task.
But hey, that's just a theory, a Bionicle theory. Sorry MatPat.