It's always existed but there was much reduced awareness of it, especially the more "middle of the road" cases. I look back at my school days and can now identify several of my class mates who clearly had Autism or Aspergers or ADHD or OCD. But back then, we just regarded them as a bit strange, which looking back is appalling and cruel, but it was how things were back then.
My best friend literally couldn't write legibly - it was impossible for anyone to read the handwriting. Rather than getting any real help, just put into the "remedial" classes with all the trouble makers. Although pretty clever, got nowhere with exams etc and ended up leaving with nothing. It was basically only the friends who realised the wasted ability/potential - but no one else cared. OK, maybe not autism, but almost certainly on the spectrum in some way. Thankfully, today, would be allowed to use a laptop or have a scribe, but back then, just cast aside.
Another lad in our class had what I now know to be clear signs, such as hand flapping, selective mutism, poor co-ordination etc., mercilessly bullied and humiliated by fellow pupils and teachers. He lived local to me so I continued to know of him through the years through his various jobs he had since leaving school - a succession of crap jobs that he could never hold onto. Except now, he's been working at Sainsburys for years, he must have finally got a diagnosis and an employer willing to give him the support he needed. Shame it's taken so long as the first few decades of his life must have been a real struggle for him.
I have no doubt that there are probably more people with such issues today, but I don't believe it's the main cause of the increase. My belief is that the main cause of the increase in diagnosis is better awareness (teachers and parents and medical professionals) so that those with the traits are caught a lot sooner and the help they need can be given.