Because they have been conned, basically. By people who literally do this for a living. And before you feel superior, watch one of James Randi’s talks (for example, this short clip) about how easy it is for the average person to get conned.
The fact is that, in a different set of circumstances, you or I could be the ones who were anti-vax. Being conned is something that can happen to anyone who isn’t prepared for it in advance!