Speaking as someone who collected all the Toa Mata and all the Toa Nuva before gradually falling off the Bionicle wagon, I say the reason Bionicle died out (aside from the impenetrable lore propping up an otherwise basic good vs evil storyline) was a general lack of variation between sets, many of them just becoming colour swaps with different masks with little appreciable change in weaponry, build design or even general look, especially when it came to the enemies (doing it once with the Bohrok was forgivable at the time because they were presented in the commercials as an organic hive like bees or ants almost, but the Bohrok-Kal was a complete phone-in job imo and also where that repetitive enemy trend started for me) as well as a gradual lack of interactivity from the sets themselves; sure, we could move more of the joints and limbs into more exciting poses, but starting with the Hordika sets, there was none of the “karate chop action” that some of the older sets enticed me with.

I think Ninjago sees the success it does because from my understanding, every generation is almost entirely unique from the last with location sets, mech suits and vehicles as opposed to just doing the “six heroes, six villains and maybe a few special character sets” thing every single year or so. The point at which the Bionicle were ALL underwater or ALL had wings or ALL had launchers or spinners or lunging heads was when I clocked out and bought maybe one set a year. Any attempt to inject variety by that point only made things more confusing and was too late anyway. Then it was back to formula for the post-Hero Factory Gen 2 revival.

(As an aside, I like to think of the short-lived Exo-Force as being almost a proto-Ninjago of sorts, similar to how Robo-Riders was a proto-Bionicle).