Bite-size: The harder it is to get something the more valuable it becomes. That is the scarcity bias.
Full-size:
Scarcity is a foundational principle of economics. When things are in limited supply, they're perceived to be more valuable than an equivalent item that's more abundant. Think if scarcity as a supply and demand problem is created in two ways: supply-induced scarcity, or demand-induced scarcity. It's also a powerful psychological tool marketers have the power to create without disrupting business economics. Because scarcity also creates hype. It could be creating "limited edition" versions of a product, or even making the product harder to get ("limited time to buy").
Another powerful force that's operating here is loss aversion: a cognitive bias causes us to feel losses more powerfully than gains (the pain of loss is twice as intensive as the equivalent pleasure of gain). As a result of this, individuals tend to try to avoid losses in whatever way possible.