First of all, this is because Bitcoin is an unregulated currency. Fiat currencies of different countries are all regulated and controlled by a central bank or by the government. And they regulate it to balance ecojomy. The money supply is actually the one they are controlling and they are trying to keep an optimum level of money supply to keep a good economy with good money circulation. And with this regulatory body absent in Bitcoin, it becomes very volatile as demand goes wild. And that's the second reason. The demand.

Bitcoin price is directly affected by demand. Simple economics. But as we all know, the bitcoin news lately is a lot which affects the demand every now and then and therefore affects the price.