Several factors in play here.
When I left teacher training college, (when dinosaurs roamed the earth.....) one of my friends (still in touch), a very talented teacher, landed what was in effect a deputy headship, unheard of so early in anyone's career! The reason was that she went to work in special education, in an institution that, until a recent change in legislation, had not been classified as a school.
Some very outdated terminology here, certainly hasn't been used for years, but below a certain ability level, children could be regarded as under the health system rather than the education system.
The staff who took care of them were nurses rather than teachers, and their remit was to care for their physical needs and keep them happy and occupied, which they did admirably; learning expectations barely came into the equation.
My friend's brief was to encourage a set-up which was more like a school, and to raise expectations of what should be expected of the children.
Another factor is the survival of vulnerable children who would, sadly, not have reached school age in earlier times. I read the autobiography of a lady, now in her nineties, who was born and raised in the East End of London. She recalled the birth of a younger brother who was "poorly"; her mother was told to treat him kindly, as he would not thrive, and he lived for about a year before this sad prediction came true. Nowadays, there would doubtless be medical interventions to keep him alive, but he would have lived with lifelong additional needs.
As for autism; I really not that old (!), but I can remember this being mentioned as a condition which was just beginning to be recognised when I was in my teens; certainly not a factor to be considered during my teacher training, It certainly existed, and in retrospect, I can think , made sense of some situations.
A saying penned by Victoria Wood and spoken by Thora Hird in one of her dramas:
"We didn't have special needs in my day, you sat at the back and did raffia work!"