although you may have read in some headlines that autism is 'on the rise' and there are an increasing number of diagnoses, I think this is a misinterpretation.
statistics looks like it's on the rise because 50 years ago we didn't know what autism was, and instead we gave diagnosis like 'idiot' and 'imbecile'.
I don't think it's a stretch to say society has always had autists, maybe rather than trying to understand why autism happens energy would be better spent in trying to understand autistic individuals.