First of all, you're mixing up immunization with eradication. The reason that deaths/instances goes down is because EVERYONE (or near everyone) is vaccinated meaning that anyone who does happen to contract it or bring it into the country/state/region has a much more minuscule chance of passing it on to others -- herd immunity.

When you choose not to vaccinate because of "personal belief", you're effectively increasing the group of people who can contract and carry the disease. The bigger that group is, the more likely it is to spread. It's a stats and #'s thing underpinned by widely accepted biomedical science. Like forget the efficacy/side effects of the actual vaccine and ingredients therein; it's about getting the # of people who can contract the disease as low as possible.

Reading the replies in this thread, it seems to me like several anti-vaxxers want the benefits of heard immunity while not wanting to accept any of the "risk" for their children because they're too precious or something. Boggles my mind how they can not see themselves as jerks.