:-XThere are simply a lot more US dollars out there than Bitcoins. Even at the peak of the most recent bubble, the total value of Bitcoins was less than $3 billion. Whereas the US dollar supply is nearly $10 trillion. If someone wanted to exchange some Euros for $1 million, the financial markets wouldn't notice a thing. If someone tried to buy a $1 million worth of Bitcoins, it would cause a noticeable spike in the price. A small number of speculators can drive large changes in the value of Bitcoins.

Another stabilizing factor for the US dollar is that people, businesses, and governments need dollars for everyday transactions. This leaves a relatively small percentage of dollars available for speculators to trade. Bitcoin doesn't yet have this same level of stable demand, so is at greater risks for speculative bubbles since such a large share of Bitcoins could be acquired by speculators.