Fear.

Fear of losing bodily autonomy and control to mandatory vaccines, fear of government overreach (easy boogeyman to paint), but most of all, fear of unknown things they don't understand (microbiology and immunology).

Too bad they will have to relearn the fear of losing their children to a formerly eradicated disease.

Of course, the bastards with a financial incentive to profit from any patsy that falls for it also perpetuate this cycle of fear and loathing.