I have ADHD and this show is awesome.
I think the people who don't like this show are just slow-witted curmudgeons of middling intelligence who couldn't think their way out of a wet paper bag filled with razor blades.
Watching the show is fulfilling on many different levels. The unique artwork, the directors idiosyncratic style of cinematography and pacing, the overarching mystery story, the astounding sound design, all the red herrings and little interesting bits in the writing, the paranormal and scientific infodumps with tons of interesting stuff they reference...
Well, I can certainly see why those who are less gifted would find this show challenging. There is a lot to keep track of, and not everyone was blessed with that kind of brainpower. Maybe they would enjoy a simpler, more straightforward show like Lucky Star.