My theory is that it’s because autists go through life not feeling like they fit in. They don’t get social cues, they don’t understand things that seem to come naturally to normal people. They feel misplaced in the world. Being trans gives them a new identity and the promise of a quick fix - it’s not that you’re just like that and you have to get used to it, it’s that you’re actually a girl and once you accept that, your life will all fall into place.
The problem is self-perpetuating, because any autist thinking of experimenting runs into autistic troons going on about how they had exactly the same feelings until they accepted their gender identity.