One of the main reasons is control. Each and every buyer can unilaterally make a decisions to sell their entire position at any moment. The overwhelming majority however do not have either the reserves or the credit to do the opposite and buy at the same magnitude.

i.e. if I have a $1 million dollar position, I have the power to try to sell $1 million dollars worth of stock, but I can only buy another $1 million dollars if I have that amount liquid in the bank or I have enough credit to borrow $1 million to double my position.

Upward movements are attenuated (or accelerated) by the quantity and velocity of credit available in the market.