Autism diagnoses grow as the definition changes (expands) and the ability to diagnose grows as well.

It's diagnosis that's growing. Which is great, because as we diagnose better we also create opportunities for early intervention and therapy. It also lets us better understand what therapies work for people and what therapies don't.

Also, common misconception about autism is that people are "getting" autism. But autism is genetic, it's not something you catch. If autism prevalence is growing it's only because autism genes are increasingly being passed by parents with genetic traits that result in autism in their children (that's how genetics works, after all).

This is getting into my own hypotheses based on time spent in the autism community (as the father of a daughter with a lot of autism behaviors / tendencies who might be considered as "high functioning" autistic) but: increase in mating success in recent times of people with soft autism signs would not be at all surprising to me.
With the advent of online dating it's much easier for people of low social function (and therefore maybe some autistic tendencies) to meet each other. For example there's a lot of discussion on a supposed autism cluster in Silicon Valley and in general links between high-achieving parents and autistic children.

In general though, people really misunderstand what autism is. I think people immediately imagine autism as the absolute worst extreme of the spectrum, instead of what it is which is a whole spectrum of behaviors which can combine in a lot of different ways.
For example: Mark Zuckerberg wears the same design of t shirt every day. Business writers (who apparently don't have a lot of interesting thing to write about) have articles that just suck his dick over it. Oh he's following Thoreau's ethic of simplicity! Oh he's such a philosopher king who realizes the need to minimize unimportant decisions! How can we emulate him in our daily lives?

If your child wants to wear the same shirt every day, guess what: they have autism. And so does Mark Zuckerberg. So maybe that autism diagnosis for your kid wanting to wear the same clothes every day isn't so scary?

I think there's widespread misunderstanding about what autism is. Regardless of whether or not it's really on the rise and why (thought there's a perfectly reasonable possible explanation in genetics there), more diagnosis and more therapy is great. Therapy, starting at a very young age, can be effective in managing any undesirable symptoms of autism. Rather than just giving a diagnosis of general retardation and throwing a kid in special ed (as in the old system) we can accurately diagnose and give people the treatments they need to live up to their full potential (e.g. guide someone from "low functioning" to "high functioning").