Because it's a commodity. The difference? You can't just print more commodity, if you need to, like you do with fiat currencies.
There is a difference between the bitcoin and a currency, however, until all 21 million coins are generated, it should be treated as such. People DO produce the currency, and that alone makes it a currency. Although, once all 21 million are generated, it will oddly enough become a sort of... reverse currency where instead of making more, the amount slowly dwindles with lost coins, etc.