The reason which i think is main is its demand in market. And demand is directly dependent on the country policies, if the policies are against crypto then the demand in that country will be very less and the price will get decrease.
There is a lot of issues affecting the price value of Bitcoin to goes up and down, like the issue on Banned, Scammer, Hacker, and the most critical one is the issue on Regulation by the country whom critical to the use of Digital transaction in the market, not to mention the good and bad news about Bitcoin (FUD). all of this indicators affect the price value of Bitcoin, and people react and they even Panic to protect their investment resulting to a huge loses in return.