According to an economic commentary by the Swedish central bank
The report says that cryptocurrencies should be regarded as a form of asset, not as money.
They mention that cryptocurrencies such as Bitcoin are not tied to any goods with a market value and that they are not issued by a national state. The report also argues that cryptocurrencies are also unable to satisfy the 'functionalism' theory - in that they are not primarily used as a means of payment, in which they point out Bitcoin in particular with that regard.
They elaborate that Bitcoin has a high transaction fees and that prices have fluctuated by a fair degree, meaning that it can't be a store of value in the way that is required to be considered as money.
However, they argue that the decisive difference between cryptocurrencies and established currencies is that the latter are "actively managed by central banks that have been given overall responsibility by law for the monetary and payment systems functioning in practice and for their being sufficiently flexible to meet the needs of society".
Meanwhile, they note that cryptocurrencies have no official issuer and that no one has any overall responsibility over them.
You can view the full report here.
Another central bank with the same argument again. There's been a chorus of these over the last few months, and it's not anything new really.
