Science, knowledge, skill, history and luck.
In earthquake country, we don't have the luxury of providing an evacuation warning for an impending earthquake. In the future, we may have a few seconds to a couple of minutes to tell people to "Drop, Cover and Hold On," but not the weeks or days that are given to emergency managers watching hurricanes brewing and headed their way.
This is not to say that having those weeks and days to consider warning your community makes it any easier. See this article, The science, skill — and luck — behind evacuation order calls, by Susan Cutter, one of the more pre-eminent scholars who work within our broader discipline. For those who have to make these decisions, it is a good article to read.
It is important to note that every state has different authorities concerning evacuation orders. Here in Washington state, there is no such thing "legally" as a mandatory evacuation order. People have the right to stay in their homes — and die (perhaps), but you can't force them out. Once people have evacuated an area, they can be prohibited from returning until it is safe. Be sure to know what the laws say for your state.
Susan addressed one of the issues that impacts people's decisions on to stay or go. That is "history" of past events and what their personal experiences have been. Should you have had a recent event that was predicted to be the "mother of all storms" and it veered off in another direction — well then, some negative learning could have been experienced. All the science in the world might not be successful in contradicting their personal experience.