This is not always because they simply don't want to get vaccinated, some people have serious reasons why they can't get them. These reasons would be called contraindications. I'll just give you a few examples of some reasons. Allergies are the main reason people can't get the flu vaccine if they have a severe egg allergy or if they have had problems with certain vaccines. Some vaccines are not even recommended during pregnancy. Additionally, if a person has any type of illness that makes them severely immunocompromised, they should not receive most vaccinations. Your doctor will talk to you before administration to make sure you do not have any of these contraindications. People often use the argument that most vaccinations are only 90-95% effective, so there is still a 5% chance with the vaccine. Some religions and countries have moral beliefs against vaccines that will prevent them from vaccinating themselves and their children