Demand. Nearly all have a supply cap or at least constraints on their inflation rate. Demand, on the other hand, is not under the control of the creators of a cryptocurrency. Hence, demand is the decisive factor.
Ultimately what somebody is willing to pay for it.
If there is anybody who still thinks that bitcoin will be at $15,000 by December 31, 2018 you're welcome to buy the small amount of my bitcoin holdings at the rate of $14,000 per bitcoin right now. After all, you'll still be making a profit by the end of the year.
What? No takers?