If your theory is that for every sale, there is an equal and opposite buy from another investor, then the price would never change. The reason there are market makers is to take the other side of the trade when no one else is buying or selling. They then try to make a profit by moving the price to their benefit. If more sellers, they start moving it down until buyers can be found. In the past 2 years, way more selling then buying, thus the price deterioration.