Thats because Bitcoin is completely decentralized, so the buying and selling of Bitcoin is totally dependent on the community. There is no panic holder for bitcoin, meaning that it will and can drop to as low as you want, however on the contrary can also rise to as much as you want.

Usually bitcoin only falls when word of FUD comes around to the light headed and inexperienced traders who sell out of panic. Also, the same can be said when people or bitcoin whales start dumping their stash.