In Bionicle, there has been a long question distinguishing Matoran Universe species that can use Kanohi and those that cannot. This is made all the more confusing when Vezon, an insane half-Skakdi, (someone who would seemingly be less able to make use of Kanohi than the average Skakdi) being able to use the Olmak; the Piraka planning without a second thought of whether they would have the "mental discipline" to use the Kanohi Ignika; as well as various jokes about species that are unable to use Kanohi being able to use masks. While these can be explained (some with more hoops than others) the differentiation of species that can use Kanohi and those that cannot seems somewhat haphazard, especially considering how Kanohi are such a hallmark of the Matoran Universe and many of the known examples of species that can use Kanohi (Vortixx, Lariska's species, Sidorak's species, Hydraxon's species, etc etc) are never seen doing so.
I'm sure there are a lot of reasons why such characters would be adversely opposed to Kanohi-usage. However, a theory detailing what could be a prominent reason that I've seen pop up in various places on the interwebs is that these species can make use of Kanohi, to some degree or another, they simply choose not to. It makes much more sense for all species that dwell within the Matoran Universe (or at least the high majority) to be capable of using Kanohi (after all, even Rahi can wear Kanohi lol), it's just that most simply don't want to. Why?
The reasoning the theory puts forth is that most denizens of the MU don't want to use Kanohi is because of their association with the Matoran/Toa/Turaga species--a species which gets all the attention, praise, and is most useful to Mata Nui (it's called the Matoran Universe, after all). The Matoran are the race that symbolically dominates the universe, and the other species are, consciously or unconsciously, jealous of them. They know that the Matoran, as a species, are upright and pure, and occupy a favored position they all secretly wish they could hold. It is why so many creatures scoff at or mock Toa throughout the universe (such as Vezok's monologue in Island of Doom, or the numerous other digs at Matoran, Toa, or Turaga throughout the series).
Kanohi are representative of the high status Matoran hold, and while some species don't care enough to avoid using them (eg those of OOMN members), most hate what it stands for, the elite role of the Matoran. Plus, as Bob pointed out when I pitched it to him, the Matoran were created WITH Kanohi whereas a lot of those other species weren't. Matoran are dependent on Kanohi. Kanohi are therefore a crutch and a sign of weakness among many of the hardier species.
Tl;dr, it isn't that the species can't use Kanohi, it's that they subconsciously are jealous of what they stand for.
--This would go a long way to taking away a lot of the complication the species lore in that regard, as well as establishing a rule as to why no species (aside from Order or Brotherhood members who were at some point served Mata Nui and the Matoran directly) regularly makes use of Kanohi where not a single Steltian, Vortixx, Piraka, Barraki, nor a single Dark Hunter (despite the many species that make it up, even former Matoran/Toa) makes use of a Kanohi.
--Additionally, it would also open up a lot of potential for story exploration and would make things less restrictive, not more, for fan stories.
--This would also end the line of inquiries asking you about the numerous species that use Kanohi.
What do you think?