Because for whatever reason people always tend to believe the bad stuff over the good. One incredibly flawed study that was later retracted by the author linked the MMR vaccine to autism and no matter how many studies proved it wrong and no matter how many times the person that wrote the paper admitted they were completely wrong and their research design was terrible, people believe the original flawed study over the mountain of research to the contrary.
People also talk about mercury in vaccines when its not been in most of them since 2001 and the amounts that were in them before that were less than harmless.
The only legitimate concern as far as vaccines are concerned is an egg allergy (or an allergy to any of the ingredients) as there is egg protein in some vaccines.
Diseases we had all but eradicated are coming back in massive outbreaks because people are refusing to vaccinate their children. Its the poor kids who are unvaccinated that suffer and given that some of these diseases can be fatal I dont understand why people wouldn't give a readily available vaccine to their child, rather than take the risk that they wont get sick. I also think that a lot of people dont realise how serious some of these illnesses can be because you never really heard that much about them for years because most people were vaccinated so they weren't a problem, now that they are coming back perhaps people will realise that they should vaccinate their children to prevent them getting these horrible illnesses.
However its also not up to me to tell people what to do with their own children. I just wish that people would do some real research before making a decision, there is plenty of impartial research out there easily available on the internet. Then parents can make an informed decision as to what they want to do with vaccines, and that decision needs to be respected, as long as its an informed one.