Well, just think about it. Lots of reasons:
A person who gets the shot can unintentionally spread smallpox to others. It can be contagous for about two weeks.
The infected spot on a person's arm can cause blindness if just a little bit gets in the eyes.
Nasty side effects: Some people get severe organ damage, some people die from it.
It's an insurance nightmare that the government doesn't want to even think about.
Having the smallpox vacine available in large quantities also means that it could be more easily acquired by terrorists. They could experiment with it and come up with something even worse.
Yikes. No thanks, I think I'll pass on getting the shot unless I really, really need it.