Therapy for autism was lackluster until the 1990s. In the 60s things like LSD and electric shock were experimented with, which as you can imagine were not pleasant and had little use. They were trying to achieve behavioral changes via pain and punishment, and the results just wouldn't come through.

Since autism is a spectrum disorder, ranging from mild Asperger's to severe, a standard of treatment that was useful for ALL autistic people was near-impossible and often regarded as a waste of time. Diagnoses were rare, and the tendency to socially-isolate made it easy for people to dismiss. Autism had to be brought out into the open, and new research had to happen.

For reasons that I don't fully understand, autism is a controversial topic. It seems it was easier to dismiss and institutionalize people, simply because we didn't how to help them. Dr. Temple Grandin (b. 1947) is a good example of someone breaking through the stigma associated with autism by the medical establishment. At two, she was diagnosed with autism. Without better understanding, the doctors labeled her brain damaged and she was put in a structured nursery school. Only when a doctor suggested trying speech therapy did Grandin make any progress. She now has her doctorate and is an activist for Autism rights. Speech therapy and behavioral therapy changed how we treated autism.

Public awareness of autism increased in the 80s and 90s. Temple Grandin started speaking publicly about autism in the 80s. Rain Man with Dustin Hoffman was released in 1988. With greater awareness, the idea that autistic people should be locked away in institutions started to be seen as an inhumane.

Source- Understanding Autism: Parents, Doctors, and the History of a Disorder Chloe Silverman 2011