I think that a big reason is also that a toa is an easy template to start from for both design and story. Say you’re new to the mocing hobby in your early teens and you’ve mostly had experience building sets. You want to make a character to represent yourself, but the creatures and whatnot don’t feel right and you don’t have many ideas. Fear not, for you have the Inika build and a ton of lore to fill with original content. Just slap some new color scheme and you have your first original character in some unexplored location on Mata Nui.
Bionicle’s lore was accessible enough to partake in it.
In my case, my selfmoc started as a dragon as I wanted something
Around that time on mocpages it was pretty popular to make MOVs of others’ selfmocs, so I tried to cash in on that and made new toa version that would be easier to reference from.
Obviously that failed, and the build was a bit fragile, so I went halfway backwards and built a simple yet sturdy lizardman. I started to write a story, so this version stuck longer, and I upgraded it over time.
Eventually I stopped the story and started another one. This prompted a new form for my selfmoc. Still reptilian, just promoted to dragonman. This is the version that has continued until today.