For BTC I think it’s been speculation about future value combined with its particular utility in being resistant to state censorship in illicit goods/services driving demand for the coins as a payment transaction system and if you want possession of a bitcoin you will either mine it or buy it. ASIC computing / energy costs / difficulty level drive the fundamental cost of what it takes to mine a btc and the market for buying BTC has to follow the mining in cost per BTC or else it drives arbitrage behavior which either drives up Bitcoin mining difficulty or the price until equilibrium is reached.
Discussion
But I don’t think many people are actually using Bitcoin itself as a tradable thing everyone is doing dollar conversion in their head and then using BTC as an intermediary. Then getting back out of BTC. In that case the price volatility is a risk. Or they are holding on speculation of future price increases and not using BTC as a medium of exchange. I think once price stabilizes (eventually) then people will have a price expectation and it can function like money pending appropriate tax treatment, Then a network effect will take place directly with the currency.