Because you aren't buying enough bitcoins! How many did you buy yesterday? None? Come on man. The reason we're stuck at the $600 range is 100% because of you. That's my theory anyways.
Well, yes it depends on the buying orders but also the fact that buying of btc will occur only when selling will occur, I mean if someone will buy at 600$ and sell at 650$ then how will the price of bitcoins reach 900-1000$ .. I think not we small individuals but the big companies determine the price of bitcoins.