i think that the reason is that traditional currencies are under the control of government, therefore they can influence the price of traditional currency, while bitcoin is decentralize currency and no government have any control on it, it is effected by the demand and supply, if the demand of bitcoin is increasing so the supply decreases which cause the price to increase.
We have seen the Chinese, Japanese, United States of America and now the European Union governments intervening every time their local currency is taking the heat. They usually do that by printing money out of thin air, nothing at all to back them. With so much supply of money in circulation in their countries, their values soon falls and inflation takes center stage. That is the world of the fiat currencies across the globe and the reason financial crisis are remains unabated in the globe.
Interestingly, Bitcoin is limited to only 21 million coins and with no one solely in control of that, it value, a true reflection of demand and supply, it is quickly assuming the role of an answer to our messed up monetary systems.