I think it has a lot to do with much better diagnostic tools and a wider spectrum of cases being considered autism. I'm not saying that is the whole reason, just a large part of it.
I am more and more inclined to lean toward the food additives/preservatives that cause the increased risk. I know of a chicken farmer in Tylertown, MS that can grow a chick to a full grown chicken in under 14 days with "special" feed. That cant be healthy for anyone.