Because its a store of value.
Its value literally comes from the fact that people who understand it believe(know)it will change the world. And, in all honesty, IT ALREADY HAS. It was the vehicle by which blockchain blazed its trail into human history....but thats all it is and this apparent when you start comparing the speed, fees, and security of using BTC as a currency. There exist faster, cheaper, anonymous blockchains that, in essence, run circles around bitcoin. Litecoin's very existence and value is a homage to bitcoin's faults as a currency.
I feel like a broken record with this point, because this realization will perpetually happen as more people pour into the crytpocurrency space. The mechanism that we should be focusing on is this:
USABLE CURRENCIES HAVE TO HAVE STABLE VALUE
Simple and plain. Can you imagine a world where you have to go constantly checking the value of your crytpo in order to decide how much to use to make a purchase? Even if that currency is relatively stable, their are minute changes in price even as the transaction is in process and how would you feel if you just spent "X" damncoin on some school supplies and now that damncoin just went down, you realized you could've spent "Y" damncoin instead.......damn.
Soooo...what now?
You have to realize that these are historic times...times that newer generations will look back on and wonder what making it rain in the strip club was like or how it felt to ignore a penny. Times like these are subject to alot of change as we are on the forefront of a financial revolution and we have to realize something critical:
Most aren't spending crytpocurrencies on things other than crytpocurrency. As of right now, they are JUST an investment vehicle.
Which means when this is all said and done, none of these currencies will change in price and their industries and use-cases will be essentially set. Many of them will erradicate others because alot of them just apply blockchain to an specific industry when applying bigger better umbrella crytpos to the same industry would be as easy as flipping a switch(or pressing enter probably)
So BTC will never die....or lie...or fly as it was intended.
It will hold whatever value it rest on as digital gold since first mover's advantage is key in this world but spending the way you would fiat is discouraged....just don't. Cashing out profits is healthy as the world is wild and unpredicatble but make sure you always hold on to your golden egg.