First, and foremost, the rise in autism rates is due to more sensitive instruments, a broadening of some inclusion criteria between DSM 4 and 5, more awareness of autism, less stigma associated with autism, services attached to autism diagnosis. I believe current research suggests these don't account for all the rise, but does account for most of it.

If I were to include additional biological possibilities, I'd include: a) birth control -> waiting to have children until a later age, more de novo mutations due to age. b) assortive mating, since some careers tend to attract those on the within and on the edges of the spectrum., c) perhaps controversially, and with the very little research on the subject, endocrine disruption from plastic waste exposures.

>Genetics does not explain steep raise in last a few decades.

as we built better telescopes, we found more stars

The steep rise in the last few decades is primarily due to increased awareness and expanded definition/diagnoses.