Because people need to eat and the grocery store doesn't know what bitcoins are?
The grocery store doesn't accept euros, or yen, or gold. But those are still money.
Well the bank does and there is an exchange.
Sure you can argue that bitcoin has an exchange and a price discovery mechanism. And it is true, just as you convert euros to dollars. The ability to interchange curriencies at market discovered prices is vital to the perception of value.
The question was why do you not consider bitcoins a currency. The simple answer is that for it to be a currency it has to have a perception of currency.