Niva's foray into numerous worlds has exposed it to various abilities. It has had the opportunity to pick what he fancies from the assortment of abilities of different gods from different races. Still, it had been careful not to be in the wrong place at the right time. Trying to witness the power of an entity so far beyond your level can be the death of you, literally. You might also fail to copy the ability after risking your life.
The sin of envy isn't infallible. A demon of envy cannot copy an ability that they can't wrap their head around. In other words, they can't copy an ability with a structure and composition too complicated for their minds. It is commendable that Nivo survived the risk and managed to copy the attack of a high god or the equivalent of a demon noble.