Only one reason really, PoW mining expense
Thats not the reason but is a big part of it. The reason the price is dropping is because one the price never really went that high, it was really just because of manipulation at the mt gox. exchange, see this report (willyreport.wordpress.com/) and secondly the price is falling because it was sold to the public as a currency which it is not, bitcoin is a digital collectible it is the equivalent of a digital baseball card and you cant price an economy in baseball cards.