Apparently it hasn't occured to anyone that autism has been very much shown to be genetic
Which when you look at the history explains a lot.
For many people they will exclaim "autism didn't exist back in my day!". Except it did. As far back as 1911. But back then it was originally believed to be a type of schizophrenia (kids with it were often diagnosed as having "childhood schizophrenia") it wasn't separated from that as a distinct diagnosis of its own till the 1980s with the publication of the DSM:III
Now typically anyone diagnosed with that...certainly up until the closure of them, would most likely have been institutionalised. And sadly especially during the 20th century many would have been forcibly steralised . So none of them were likely to ever have kids or pass on their genetics.
Another point would be that women especially, certainly prior to the early 2000s weren't even thought of as even being able to have autism. This was due to the major gender bias and the fact that also every study on autism to that point had ONLY been focused on males. As we now know women can be as much affected but we also know that women/girls are far more likely to be able to mask the symptoms, so combine that with the lack of medical knowledge and the very limited chance any girl back then would even be assessed means you have a whole generation of women who most likely always had autism but were never ever considered for evaluation or diagnosis.
Which leads to my next point, most of those women born in the 80s/90s are the ones currently having children. So if they didn't know they had autism and it was a partt of their genes many women have unknowingly passed that on. Obviously as there has been 30+ years of research in that time, schools etc are far more aware of it, and far more likely to screen and refer to assessment. That has then, as obviously a parents observations of the child are a key part of the diagnostic asessment has led to many parents whos kids are diagnosed to suddenly question their whole lives as they realise they took have the same issues and struggles as their child.
And then you take into account the fact that typically the age at which people have kids is ever increasing and one known fact is that as men get older the quality of sperm decreases and as such older men are more likely to father children that had autism.
So if you add all of those together it becomes quite apparent why there is seemingly an "increase"
And it's fuck all to do with vaccines