To be considered a currency and have any use or application in economic markets, a currency must be exchangeable for goods. Bitcoin isn’t.
Many retail outlets that originally accepted Bitcoins have changed their minds. In a BBC poll in 2017 among ten businesses in London that began accepting Bitcoin, four had reversed their decision and another two said they still accepted it but almost never received payments in Bitcoin.  If Bitcoin cannot gain traction as a vehicle of exchange for goods, then it cannot be considered a currency. Part of the reason why Bitcoin fails as even a basic currency is that without the backing of a central bank, its value is susceptible to wild fluctuation. Additionally, it is not very liquid. It cannot be carried around in a wallet like cash, and it cannot be integrated or stored in a conventional bank account.
Just because Bitcoin is not used in the same way as a government back currency doesn’t mean it isn’t the future. Bitcoin has a future as a store of value. In a world where coronavirus, bank collapses and corrupt governments can cause turmoil in currency and capital markets, Bitcoin could emerge as a prominent vehicle for storing capital, immune from prying government eyes, global economic shocks and individual corruption.
A currency is something that is exchangeable for goods or services.
[P1] Currencies are items that have a widely agreed-upon value and can be traded for goods and services. [P2] Bitcoin cannot be traded for goods and services. [P3] Therefore, Bitcoin is not a currency.
Rejecting the premises
[Rejecting P1] Currencies are a vehicle for storing value. Bitcoin stores value. [Rejecting P3] Therefore, Bitcoin is a currency.