Dalrymple fathoms a guess:

Perhaps people felt that to immunize was to interfere sacrilegiously with the course of nature, and that people, especially children, had the duty to die of infectious diseases just as Nature “intended.” Perhaps they felt that, if it worked, it would allow the survival of the unfittest. At any rate, few medical procedures have been as persistently, minutely and fervently examined for harmful effects as immunization.

Nevertheless, he notes one recent study that may embolden the anti-immunization crowd. Read it here.