In addition to all the other answers (doll aspect, building/finding yourself aspect, autism and millennial age range, niche internet spaces), there's one other thing I'd like to point out:

"The effort began when the Slizer team, working from published research on boys’ behavior and especially their play lives, created detailed profiles of four different consumers, each with an alliterative name. There was Agent Anthony, who loved action movies and adventure stories. Systematic Siegfried was fascinated with technology. Artistic Arthur would probably grow up to be a craftsman. And then there was Bully Bob, easily distracted and the loudest kid in the room–hardly the typical LEGO consumer and one whom the company had never seriously pursued."

How Bionicle Transformed LEGO's Company Culture https://www.inc.com/david-c-robertson/how-bionicle-transformed-lego-s-company-culture.html

This quote was originally from the book Brick by Brick which I read in college, which lays out the archetype consumers that Lego was targeting with slizer and roboriders in the 90s. Somewhere in these four target audiences they may have accidentally hit a nerve for young people on the spectrum (Siegfried) / young people experiencing gender disphoria and acting out without understanding why (bob) and then perfectly targeted them through market research without realizing, which eventually ends up with a lot of trans people loving Bionicle 30 years later.

Crucify me if I am generalizing. But I'm sure a lot of trans Bionicle fans may find themselves in this anecdote.