The tragedy of the commons is a factor here; the Tragedy of selfishness?
"In only one respect is my message the same as the anti-vaccine activists: Educate yourself. But while they mean “Read all these websites that support our position”, I suggest you should learn what the scientific community says. Learn how the immune system works. Go read about the history of disease before vaccines, and talk to older people who grew up when polio, measles, and other diseases couldn’t be prevented...."