Also, with all the risks children go through every day, why are vaccines the unacceptable one? They're probably more at risk riding in the car with you than they are getting an adverse reaction from a vaccine.
The short answer is people are stupid about cost-benefit analysis.
The long answer is the various ways people can be stupid in this regard, including:
When evaluating two courses of action (not vaccinating vs vaccinating), people who are uncertain tend to default to doing nothing.
Related to the above, parents are much more likely to feel guilty over side-effects of vaccinating than those of not vaccinating, because it's framed as "a choice I made" vs "something that happened". This still applies with things that aren't actually caused by vaccines as long as the person believes it was (autism being the obvious example).
Injecting things is scary because it's "artificial", disease is less so because it's "natural".
People are way more tolerant of risks that bring convenience (driving) than risks that don't or are mildly inconvenient, because the risk is abstract and the convenience is concrete.
And other stuff I can't think of at the moment.
