without short selling market down moves become more crash-y - short sellers buy (to realize profit) as the stock goes down which tends to support the price. wit

without short selling market down moves become more crash-y - short sellers buy (to realize profit) as the stock goes down which tends to support the price. without buyers, an out of favor stock might keep going down in price and have maybe no willing buyers! and that also affects spreads, because market makers hate volatility. oversimplified but real