I agree with Amber and PolterGoose. I believe it has always been here, but viewed differently . 200 years ago, people with classic autism were "possessed", 150 years ago, "the village idiot", 100 years ago, they were locked away, and that remained the case until well into the latter half of the last century.
Those on the "milder" end of the spectrum were odd and labelled in other ways, either with psychiatric diagnoses or derisive terms, and expected to fit in or go away. In many cases, the pressures to conform intensified underlying mental health issues and these people ended up institutionalized or dead. Vincent van Gogh is a perfect example of this.
Now that the existence of a spectrum is widely accepted, we can see that while those on the "classic autism" end of the spectrum are profoundly affected, and support for those individuals, while it can be challenging, is widely available. Those on the "HFA/Asperger" end of the spectrum, on the other hand, tend to fall between the cracks. Either they have become so adept at masking the effects of their differences that they can fit in, but it takes such a toll on them as to leave them physically and emotionally empty, or they "stick out" and are ostracised and blamed. Individuals are still profoundly affected, regardless of their place on the spectrum, just in vastly different ways.
Grouping all individuals on the autism spectrum under the same umbrella is akin to saying all amputees are the same. An amputee who has lost a leg and one who has lost an arm both have distinctly different needs, and are both profoundly affected, but in very different ways.
And, as the last comment (at least in this post - I talk a lot)... I do know of one (and ONLY one) family who claims their child is autistic, but the kid does just lack structure and discipline. He controls his parents, is rude, talks back, curses and is manipulative... possibly a sign of some pathology, but definitely not autistic. The parents refuse to accept any help for him or themselves but want to get whatever financial gain they can, and impose nothing that even vaguely resembles discipline. When the child is here without his parents, he is entirely different. Plays cooperatively, is polite and respectful, follows the rules and our home schedule. We have had others who know him in his home come here and say the same thing - that as soon as his parents are not around, he is a delight to be with, both at school and out in the community. His parents, however, swear he has autism (but won't get him involved in any therapies, or attend any support of their own). They do, however, keep pushing to try and access thousands of dollars in annual funding (that they have been turned down repeatedly for, because they can't find a single professional who will put anything in writing to support their claims, even in private assessments).
Sadly, because the spectrum is not black and white, there ARE those who will seek to abuse it.