I would say generational memory is a factor, if you've seen an uncle or a grandparent who'd had polio, or you're old enough that measles was a thing in your youth, and you've seen these ailments disappear over your lifetime, the lies and FUD will have a much harder time taking root.

If these ailments are completely abstract in both their scope and personal effects, it's easier to be convinced by emotional manipulators. Especially if you're part of the... let's say low empathy population.

It is not lies if Mr. Kennedy actually believes vaccines are harmful. Idiot, yes. Liar, no.