First of all, nobody says Bitcoin is (only) a currency in a sense you think.It's exactly those huge fluctuations, control by big players and unreliability why BitCoin can only dream of becoming a future currency. For now it's just one of those fun commodities on which lucky or well-informed people can make money, and the rest will lose money (because you can't win unless someone else loses). For something to become currency, it has to be stable. Nobody likes hyperinflation except a few sharks at the top.
Second of all, being stable is not a requirement of a currency. There are other more important aspects like being fungible, divisible , ease of use. All currencies fluctuate in value unless they are artificially pegged which is usually not a good thing and sustainable in a long term.
Right now bitcoin is considered more as a commodity, a store of value. It is also used as a currency/payment method but this is not its main appeal right now as there are better (faster and cheaper) crypto tailored to that use.
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