My take on the pop-science of autism
It is because humans are under evolutionary pressure to get smart, fast. If you take people who are extremely smart and let them have kids, some percent will get autism. Just enough autism and you're qualified to be a PhD or a software developer. Too much and you're rain man & live in an institution.
There is no environmental factor, other than an economy that has started to reward engineers with high paid jobs that allow you enough money to have kids.