Well, it turns out it was. The movie companies approached by Lego must've been anxious about releasing a movie without a human protagonist for a reason, so I'd assume the early '00s weren't that big on robots either.
As I said, Chima featured anthropomorphic animals and they're a commonplace in children's fiction. They're also a much more relatable and down-to-earth concept than alien robots. Try recommending Zootopia to someone, then try doing the same with a Bionicle movie, and see the results for yourself (sure, I know it also might be the matter of the actual quality of the writing).
Sure, there are robots whose design is cute and will probably appeal to a large number of people of any age. Like this. Or this. However, robots like this or this (which are much closer to the Bionicle visual language) either accompany the humans or are their enemies. The Bionicle movies tried to make the Matoran and the Toa look cuter and more expressive by giving them large eyes and mouths but many people I knew at the time insisted that the characters were still a bit ugly and alien-looking.
And I'm not trying to discourage anyone from doing their concept art of Bionicle as a System theme (they're not my cup of tea but they're cute and all, I guess) but the notion that some of you (not all of you, though) seem to have jumped from 'G2 was a flop, don't expect G3 anytime soon' to 'G3 will do just fine, just make it a System theme' is what I find ridiculous. First, you're setting yourself up for disappointment here. Second, as I said, you seem to ignore many important factors that contributed to the success of themes such as Ninjago or Nexo Knights.