Scarcity
People tend to want things as they become less available. This has led advertisers to promote goods as "limited availability", or "short time only".
It has also been shown that when information is restricted (such as through censorship), people want the information more and will hold that information in higher regard.
Items are also given a higher value when they were once in high supply but have now become scarce.
Read more about this topic: Influence: Science And Practice