The exchanges are part of the market. If the market doesn't have the supply, but a bunch of people are wanting it (demand). The price goes up. There needs to be more supply on the market, with less demand to go down. It's possible to drop if demand dries up, even if the supply is low or high. But that's usually with physical items that age out of use or failed projects or companies. It really depends on what it is. Like computer ram that's 10 years old, there is still a small demand, but the supply is also low and no more are being made, so it can be more expensive than new ram.