It seems to me that over time we've simply traded off one set of carcinogens for others. Wood smoke has at least 11 known carcinogens, and not that far back everyone used fire to heat and cook with almost constantly. Sunscreen wasn't commonly available that long ago either, and heavy metals were in common use for everything from jewelry, ceramics, eating utensils and water supply systems. We do have a lot of new synthetic carcinogens in the system now, so the threat persists, but I don't think it is any greater per capita than it has always been. What has improved greatly is our ability to diagnose the disease and raise the awareness of the risk. That in itself most likely contributes to the greater number of reported and confirmed cases than what was being done even a hundred years ago.

Genetic disposition towards certain conditions is a known property of our existence. Just as with certain genetic traits making some individuals more susceptible to cancer, heart disease, or alzheimers, so to it is most likely with autism.