I can safely assume they don’t mention BIONICLE because the LEGO group is a company geared entirely towards selling toys. Never under any circumstance would LEGO want to take attention away from their current product.
It should also be noted that a very large majority of the people who originally worked on BIONICLE are gone. Having most likely moved on to other opportunities outside of LEGO. The people you don’t see now making all those cheeky references to BIONICLE & GALIDOR are employees of LEGO who grew up playing with BIONICLE. Even then, the only references they can do are whatever’s in the budget for the set they’re designing.
Without a doubt there’s probably any number of people within LEGO who don’t care for BIONICLE, but doesn’t it seem counter intuitive for a company to disrespect its own intellectual property?
Now, finally, regarding Takanuva’s cut appearance in The LEGO Movie 2.
I don’t know how or why he was in that concept art, being that it’s concept art I assume it is used to generate ideas and anyone can draw anything there. Don’t assume someone at LEGO slammed their fist on a table and demanded Takanuva not be in the movie.
It is my assumption that when considering what would fit into Rex Dangervest’s niche of of that edgy teenage demographic, of course BIONICLE would be considered.
I’ll even let you in on a secret. Do you want to know what Takanuva would’ve looked like in the movie?
Here you go.
Some people assume that this is a mech suit, and you can see Takanuva’s legs sticking out on the bottom there. Personally, I don’t believe that. Maybe if G2 had continued, maybe this prototype would have CCBS elements in there. One thing to bear in mind though is that CCBS is gone. Retired. All those parts aren’t being produced anymore so making this Takanuva out of bricks and technic shouldn’t be mistaken as any sort of dis towards BIONICLE. If Takanuva had been in the movie there’d have simply been no BIONICLE elements to make him with available to the set designer.
I personally assume that this is just something along the lines of a drone, and the name Takanuva is in the same vein as The LEGO Movie’s and The LEGO Batman Movie’s BIONICLE references. Just that, a reference. I.E. a blink and you’ll miss it easter egg thrown in by people at LEGO who love BIONICLE just the same as you and I.
Why wasn’t it in the movie? Well, it’s a movie, and they might’ve wanted to focus solely on the raptors. Having Rex have robots and mech suits and spaceships would dilute the raptors, which have much more joke potential. Also a name like Takanuva would be weird to put in the LEGO Movie 2, obscure reference or not.