Because there are relatively few buyers and sellers in the market and the amounts of transactions are small, so one big mover can shift the price a lot.
According to some blockchain site[0] the average transaction volume for BTC is something like $2.25m USD equivalent
For comparison, on an average day, SPY[1], which is just one security (albeit a popular one) trades something around $17,160m (e.g. $17.16b).
[0] https://blockchain.info/charts/estimated-transaction-volume-...