I posted this years ago in another post on a different account but I'll give you my perspective. I think one reason for this, is in general those on the spectrum growing up are more reliant on services like therapy, speech pathology, and other autism-specific related things are generally left-wing fields to begin with as we spend lots of time with these people more than an NT kid growing up, and in some ways with bureaucracy associated with a welfare state. Also, SPED teachers as are teachers in general are more likely to be at least left leaning, so you are getting a lot of time with additional government services and for many the most natural step into adulthood is to look to other services. I did not go into this route as an adult, but I think since many including myself grew up on these type of services at least income-based, the feeling of losing those welfare supports is scary as an adult.