In short, increased difficulty of mining makes it harder to increase the supply of a cryptocurrency.
In the same time the rising demand, combined with the limited supply, is supposed to result in increase in price.
Utility of any cryptocurrency is one of the key factors concerning its price.
If you solve a problem with its help or you can use it as a payment instrument then itʼs more and more wanted and the price increases.
When talking about Bitcoin we should have in mind that itʼs used for payments, itʼs difficult to earn it and this process consumes a lot of energy.
Therefore, the price appears to be reasonably high. In the case of Ethereum it is another story.
The currency is valuable because itʼs also based on open source and allows developers to write their DApps.