Because nowadays children that are 'different' to their peers, in whatever way, are labelled with one syndrome or another. In the past they would have been called other names( good and bad) perhaps to mark the difference, or maybe the difference would have just been accepted as part of their personality. Doubtless some of these children were autistic.

I can remember an aunt, who's behaviour would nowadays fall into a spectrum of one kind or another. She lived her life with the support of her family and no medical diagnosis/supervision.

There were killer childhood diseases then and adults lived shorter. Treating physical illness was more pressing than investigating behavioral/mental illnesses. Hence the asylums being places where the poor souls lived their days in 'secure' surroundings. Medical breakthroughs have brought us cures to some physical illnesses that in days before were time consuming in the nursing of.

The extra time/money can be spent thankfully on researching many areas within medicine. And the result being we now have stats to show the numbers of people with autism, motor neurones, parkinsons etc etc etc. To assume they didnt exist before they were recognised and named I find hard to believe.