Tonnes of reasons. Parents being older before having children, spending your life surrounded by biologically active plastics (Hello, BPA, and its yet-to-be-banned cousins), changes in diet, Jupiter being in retrograde, etc, etc, etc.
Any one of them, even the wild-ass nonsense ones seem more plausible to me than one fraudulent study written by a doctor who wrote it in order to... Sell autism testing kits and litigation.
You should put less trust in proven, shameless liars, who have a simple explanation for all that ails you, that they will sell you a cure for.
It's not religious faith, but you're going to need a lot more than a shoddy argument.
The argument, by the way, is so shoddy (by failing to be rigorous about both diagnosis methodology, and of differences in environment) that we could trivially use it to conclude that vaccines actually reduce incidences of autism, if we look at America as the undertreated high-autism, under-vaccinated baseline, and at Europe as the properly treated gold-standard... But I don't think that's what you're interested in exploring, no?
There may be a reason why this question attracts so many frauds, quacks, and fools. I postulate: the people who run well-made studies, that we can actually draw meaningful conclusions from aren't finding the answers that the frauds, quacks and fools like.
We know vaccines aren't the cause, because large-scale studies have been done that show that autism rates aren't higher in vaccinated children. This leaves us with basically every other possible cause, including not just better diagnostics (I personally find that explanation highly plausible), but also changes in nutrition, effects of some sort of virus, pollution, microplastics and everything else under the sun that's changed in the last decades.
(Did you have any idea other than "anything but the vaccines"?)
It's been decades; the kids are the study. Also, they're no longer kids. Do people who have taken the vaccines report higher rates of autism? It would be relatively simple to check for that.
> Did you have any idea other than "anything but the vaccines"?
Increased awareness of autism, increased medical attention on kids at large, increased medicalization of societal deviance, less socialization, less parental time, environmental factors (more CO2, more exposure to pollutants, more exposure to forever chemicals), dietary factors (more sugar, more carbs, more fat, microplastics), derived factors from any of the above (more screen time, earlier occurence of obesity and diabetes, higher prevalence and normalization of societal deviance).
All of those are as likely as vaccines; which is to say, extremely unlikely. We can test for them. Or we can keep testing this one thing we've already tested for, over and over, until as statistically expected, a study eventually happens to agree with your preconceptions about vaccines, proving the all-encompassing conspiracy of $cienti$t$ and $tati$tic$ and error margin$, and plunging us into another two decades of unvaccinated idiocy.