There are two types of anti-vaxxers. The selfish ones do generally believe in vaccines but as there may be a chance of some side effects in some people, they will use the herd immunity and not vaccinate their kids since they are so special. They think everyone else should vaccinate.
The hard core anti-vaxxers genuinely believe vaccines are just a big-pharma conspiracy and either don't work or have so many side effects it will hurt children more than the disease. They think no one should vaccinate.
1) The Apathetic who can't be bothered with understanding the the risks of vaccine as the chances of catching something is so remote why bother
2) The Selfish who believe there is a risk from vaccinations and expect the herd to protect their kids
3) The Delusional who think Big Pharma created vaccines to make money and they do more harm than good
Asking people to be honest why they don't vaccinate is a challenge. Perhaps a public register of parents who don't vaccinate their kids would be sufficient to shame people into doing it.