At the risk of getting more off-topic, I'd be happy to try to explain; which part made no sense?
The main ideological crux of the people I've met who are in that sphere (I think I'm safe in generalizing here) is a distrust of science. That is, with respect to vaccinations in particular, distrust of the NIH, WHO, doctors, researchers, the CDC, the FDA, the medical community, publications, etc., etc. They choose to believe their own "common sense", and/or whatever the propaganda in their sphere spreads, rather than the science (note: I think that's an objective characterization, at least among the people I've talked with who maintain those beliefs, which I think they would generally agree with). That's what I meant by agreeing to disagree about the veracity of science.
And as noted, it's a free country, and for better or worse people can choose what and who they wish to believe. People also have an almost unlimited capacity to rationalize what they observe to conform to their world views, as recent history has demonstrated (which makes altering hardened beliefs difficult, irrespective of otherwise unassailable evidence). 'Tis why the country is the most divided it had ever been atm (see note about that human condition being exploitable).
No blame, mind you; just facts.