While there are some significant examples out there which more than manage to defy this trend, speaking much more broadly I’d have to agree with LegoDavid - if you’re marketing to kids, your odds will be better with TV or movies than with books. That said, my original point wasn’t about what would have been viable for Bionicle during its lifespan, but what would make it more accessible to pop culture fans now. Having all the story content in novel form, and having those novels be more accessible to older fans, would have impacted that significantly, I feel.
As far as Direct-to-Video goes MoL did very well - well enough that LoMN nearly went cinematic, I believe I’ve read before (either Faber or Greg said something about that). And the directors and producer have said that the plan was never to make a trilogy - each film began production only after Lego saw how the previous one performed - so that suggests the movies did well for what they were, financially speaking.
When you take a step back and remember they were made in under a year each, it’s easy to be a bit more forgiving on some of the flaws they had and I think they did a respectable job. That still doesn’t make them good examples of Bionicle’s finer moments, though. 