Bitcoin is a currency, because it can be readily transferred from one person to another, for something else. But for Bitcoin to become a mainstream currency it needs to be stable and not so volatile. A merchant can't sell you something for $100 worth of Bitcoin and then have that value go down to $70. For it to become a mainstream currency it would probably need to be adopted by a government, backed by them, and that could give it some stability. But what strong government is going to do that, especially when in the beginning is not going to be stable. Will probably see it pump Bitcoin to astronomical levels, only to have it crash hard, hurting the country.