It doesn't meet the requirements of a good currency. There is hyper inflation and they have redenominated it a few times now and are planning on doing it again.
Is a boat with a huge hole in its hull a good boat? Technically it is probably a boat, but it isn't what one would consider a good and desirable boat and it doesn't allow one to do what one can usually do with boats.
This is the same with BTC. You can call it a currency, but if you use it as a currency it doesn't currently exhibit the properties that one wants from a currency.