As per my observation on Price of BTC/USD totally depends on market. Because when market collapse it goes down and when in upward trend it rises. See History periodically you will notice.
Why is the price still dropping?
Main reason of falling rate of BTC is halving. People still thinking that after halving price of bitcoin will goes down as it will hard to mine as rewards will be half than now!
People are selling their bitcoins so as per demand and suply formula price of bitcoins is going down.