Like Mario, Crash Bandicoot etc are similar kid friendly platform games but have nowhere near the same amount of autists in their fanbases. What gives?
I think it's his flesh-colored, human-like arms. So many blue cartoon characters have arms that are, for example, blue. But Sonic, despite being a blue hedgehog, has non-blue arms. As a person who also has non-blue arms, and autism, that really helps me to feel a deeper connection to Sonic than I do to, say, Mario. Mario wears overalls that cover his entire body, including, crucially, his arms, so we really don't know what color his arms are underneath. They could be purple, or orange, or blue.
Sonic is NOT autistic. To be fair, you have to have a very high IQ to understand Sonic. The humour is extremely subtle, and without a solid grasp of theoretical physics most of the jokes will go over a typical gamer's head. There's also Knuckles' nihilistic outlook, which is deftly woven into his characterisation- his personal philosophy draws heavily from Narodnaya Volya literature, for instance. The fans understand this stuff; they have the intellectual capacity to truly appreciate the depths of these jokes, to realise that they're not just funny- they say something deep about LIFE. As a consequence people who dislike Sonic truly ARE idiots- of course they wouldn't appreciate, for instance, the humour in Tails' existential catchphrase "Wubba Lubba Dub Dub," which itself is a cryptic reference to Turgenev's Russian epic Fathers and Sons. I'm smirking right now just imagining one of those addlepated simpletons scratching their heads in confusion as Robotnik's genius wit unfolds itself on their television screens. What fools.. how I pity them.
And yes, by the way, i DO have a Sonic tattoo. And no, you cannot see it. It's for the ladies' eyes only- and even then they have to demonstrate that they're within 5 IQ points of my own (preferably lower) beforehand. Nothin personnel kid
Autists perpetually hold on to things they found cool in childhood because it makes them feel safe. On a completely unrelated note, I love warhammer 40k.
I think it was the right combination of being in the early 90s and having a wide assortment of cartoon media that made Sonic a draw for future autists. Sonic is also kind of a gateway to the furry fandom, which is also an autism magnet.
Supposedly Sonic appeals to those with the 'tism because the levels are very linear and patterns of loops and walls are soothing/easy to understand. Similar to how many of them tend to like trains and the tracks they follow.