Most likely because little new fiat money is coming in. Same people buying/selling.
Once the FinCEN thing settles down and it becomes easier to aquire bitcoin in the United States prices will go up.
https://billpayforcoins.com/index.phpServices like that, assuming they survive and aren't shut down due to paperwork issues, will help make bitcoin a viable option in the United States and more mom & pop type businesses will start accepting bitcoin payment - peaking the curiosity of their customers and thus increasing demand.