If it was easier to buy and sell them they might. I know people who are interested but due to the difficulty and complexity of buying/storing/selling they havnt yet.
If there was a way for crypto-currencies to handle credit, I think that would greatly increase it's likelihood of replacing real world currencies and central banks--going mainstream. The money we use nowadays is transferable credit and central banks can produce enough to meet the demands of the economy. Crypto-currencies on the other hand are more like gold: there's a limited supply that doesn't meet the needs or demands of the economy.