Apologies for the brevity and avoidance of in-depth discussion in this answer, for simplicity's sake.
To understand Bitcoin and its failures, we must understand what a 'currency' is, and why it is important.
Prior to currency, people bartered for goods - I give you 4 chickens, you give me a goat. What if you don't want my chickens? How do I get my goat? Well I could trade my chickens for your neighbors calf, and trade you the calf for the goat, etc. But there are a few problems with this system:
(1) It takes time for me to 'shop' my chickens around to trade; [They are a poor 'medium of exchange']
(2) Chickens, goats, and calves only hold value while they are live / edible. [They are all poor 'stores of value']
(3) How much each person values a chicken, calf, or goat will differ based on even what they want to eat that week [it is a poor 'unit of account'].
Coincidentally, those bolded items there represent the 3 qualities of money. From a practical standpoint, you can see some of these having been met by a quasi-bartering currency, such as cocoa beans (South America) or pounds of salt (Rome), where the 'money' itself had a definable practical use. You can also see these principals with rare metals such as gold, which were the first minted coins. Now in any of these cases, the 'value' of the currency truly is in the eye of the beholder. If someone in prison is going to accept a pack of ramen noodles as money, it is because either they will use its inherent value, or simply they believe in the market's general valuation of that ramen, and have faith that they will be able to trade it to someone else down the road.
Metal coins technically had inherent value in their ability to be melted down for jewelry, but there are problems with this form of currency - metal content can be faked, new mine sites adjust inflation in non-controllable ways, and ultimately modern economies are unrelated to the handful of gold mines still in existence.
Modern economies have historically printed currency 'backed by' gold, to get around some of the physical problems of a metal-based currency [meaning for every dollar bill printed in the US, there was a speck of gold that was held by the US government, theoretically redeemable].
But what is that 'true' value of a dollar 'backed by' a speck of gold? For an inherently valuable money like salt, this 'market valuation' is not as much in question. Someone will always need salt, and therefore if salt is rare / hard to produce [which it no longer is], then owning salt will give you future market bargaining power. But what if your money is a minted coin, with a value above the raw ability for jewelry? Well you're in luck if that money is distributed by your government, because they will accept it for something tremendously important - to pay your taxes.
Proponents of gold [and, amusingly to me, bitcoin] dislike the fact that gold no longer backs the US reserve, but frankly, its true modern value is as settlement of your tax debt with the government. And that usage today is the same whether the government backs it with gold, or by proclamation (value by 'fiat').
Watch out, the below contains a lot of opinionated comments.
So why does almost no one accept bitcoin as a currency? Because it has no inherent value, and it cannot be used to settle a tax debt. And because it is insecure [no authority to assist with payment issues]. And because it does not 'play well' with a modern electronic economy [try explaining to a mom and pop shop how to set up a bitcoin wallet securely, and confirm their payment receipts on the blockchain, vs just paying Visa 3% and forgetting about it]. And because it is deflationary by nature [who wants to pay someone 10 BTC for a pizza worth $20 today, when it could be worth $200k in 5 years?]. And because it is volatile [who wants to accept 1 BTC today for $20k, when it could be worth $6k in 3 months?]
So why does BTC have a value at all? Because it avoids 'know your client' banking requirements allowing money laundering or black market activity. And because people love to gamble. And because it lends itself an air of superiority to its users. And because it makes some hopeful claims about the future (allowing the 'banking of the unbanked').
For the interest of the reader, note that today, BTC was valued at between $6500 and $7000 USD [difference due to various bitcoin markets with massive price spreads due to technical issues (look up 'Tether October 16 2018' for an example) not worth getting into here, but more than anything else, being a signal of the volatility and manipulation of the market].