The market does.
The cryptocurrency market runs of speculation.
It's basically a function of how much I think someone would be likely to pay for a digital asset in future.
One of such indications is:
1. how much utility the project behind the cryptocurrency provides.
2. how much demand there is for the cryptocurrency now.
3. how many people trusts it.
In the case of bitcoin, for example, (1)it's a means of exchange and store of value. (2) there is a good demand (as opposed to other coins) (3) it's trusted by almost everyone in the cryptocurrency space
The price of Bitcoin Is determined by supply and demand.
As we all know by general economics when the price goes up and the commodity can't keep up with the demand and price goes low when the demand is low and supply is in abundance.
With the supply of Bitcoin is limited to 21 million ,once that mark is reached there wont be anymore coins to be mined by miners. Till date their are 18 million BTC in circulation, that is about 80% of it is has been mined and available to the public for use. So with supply being limited and ever growing popularity ,the price of BTC is speculated to moon.