There are many reasons, one being safety. trying to leave is when many victims are killed so its a safety issue that has to be planned out well. Additionally many abusers have separated the victim from jobs, family, and finances so they will end up being homeless. Another reason is children and pets. In many states if the victims leave it can be construed as abandoning the house of children or kidnapping the children if they take them with them. Pets are used - abusers threaten to kill the pets if the person leaves and often shelters cannot accommodate pets. It's very complicated and laws do now always help the person leave safely.