Some reasons I can think of:
- Vaccines are usually injected, injection makes most people uncomfortable. Most of them get over it, but it can be panic inducing for a small number. Among street drug users, injection is usually considered extreme, as in, the thing only true addicts do.
- When you get vaccinated, you are fine, and then you get a few unpleasant side effects. That's the opposite of the usual drugs you take when you are sick and make you feel better.
- Sometimes, vaccination is made mandatory, many people don't like to be told what to do with their bodies, even if there are good reasons for it.
- You never know when a vaccine have saved someone. Someone didn't get sick, but is it because of the vaccine, natural immunity, luck, or simply a lack of exposure, maybe he will get sick eventually, you can't really tell, and you don't really notice when something doesn't happen. However, when a vaccine hurt someone, or failed to prevent the disease it should protect against, that's very noticeable.
- The most successful vaccines are now often considered useless, because the disease they protect against is so rare, why go through the discomfort of the vaccination when it is so unlikely for you to get sick either way? It is a tragedy of the commons, as the reason the diseases is so rare is because of widespread vaccination.