Increases in public awareness of disease outbreaks (including polio, measles, rubella and others), along with the desire to protect children from future epidemics.
Anti-vaxxer thought is nothing new.
Jamie Attenborough

This, along with reason #4 below, is a critical element in public acceptance of “science.”. To most people, whether we (in the sciences) like it or not, “science” is a combination of a “religion” and a “tool”. In the former dimension, people “believe in” science because it is and must be “right” in not just factual but also a “moral” sense. In the latter sense, people accept science because it addresses and provides solutions for problems whose gravity they understand.

The former, the quasi-religious acceptance of science, is a dangerous trap that too many people — science advocates as much as its enemies — fall into. You “believe” in something because “faith” makes up for the lack of sufficient “facts.” If science rests on consistency of internal logic backed up by empirical approach to epistemology, there is no room for “faith.” One can’t tell people to “just believe in” what scientists have to say. The empirical basis for accepting science, as far as most people are concerned, comes from the fact that science provides solutions that people can see to the problems that they are obvious and serious. But the latter is increasingly difficult: we no longer have smallpox epidemics; polio is nonexistent outside a few pockets in the developing world; many formerly dreaded diseases are hardly visible and their gravity underappreciated — precisely because they are so rare. So we increasingly wind up putting science up on a quasi-religious pedestal where it looks like a false idol — especially to the “infidels.”

This is, I think, just plain wrong (morally) on all sides: that we wound up having to sell science like a mystery cult is the failure of science education that skipped the parts about consistency of internal logic and empirical approach to epistemology and doodled on about the “right answers” and how “cool” they are. If science looks like a mystery cult, even if that characterization is (factually) wrong, it is hardly shocking that many people have trouble believing. It is equally shocking (not) that some social elites should try to take advantage of them.