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.
Market is Volatile, price value of Bitcoin may goes up and goes down again dependent on the Volume of investment put in by the investors in the market, a lot of indicators that affect the price movement in the market, like the issues on Banned, Scammer, Hacker, and the issue on Regulation by the government financial institution whom critical on Bitcoin digital currency investment in the market. The other thing that might affect the price of value of Bitcoin is the Negative Speculation posted in this forum or in other, even the negative news about Bitcoin, market reacts on both positive or negative news item about Bitcoin.