Because especially compared to other assets like gold and stocks, liquidity is a lot lower with bitcoin. Stocks and gold has existed for so long already that a lot of people and a lot of machines are trading them back and forth a lot of times on the daily. As with bitcoin, it's just more than a decade old, and is very very easily affected by news and rumours knowing that a lot of people that are buying and holding bitcoin never traded anything in the past before.

Assuming you're referring to prices from sites like Coinmarketcap, the Coinmarketcap price of bitcoin and other cryptos is the average of these coins' prices on exchanges. If you're talking about specific exchanges, then prices of coins are decided with the latest executed buy/sell trade offer.