Well, quite simply because it's easy to fall quickly when you're very high, and conversely to rise very quickly when you're very low.
With markets at very high levels like we are experiencing at the moment, there are fewer and fewer new buyers and less and less cash to invest. Everyone is already in the market. The market is rising less and less quickly, it is settling down, then people start to feel nervous. There are fixed stops that are triggered, there are other smart guys who start to think that it is becoming interesting to play on the downside and the machine goes wild. All the guys who arrived last, the suckers who don't know much about the stock market, panic. They bought high, they have just lost 10% in a week and they are starting to sell at a loss. Since there are many in this situation, it creates a mass effect. And it continues until, much later, more savvy investors, opportunity seekers, arrive to buy at a good price. Then the market can start to rise again very strongly.
But you're right, markets fall much faster than they rise. Apparently man panics faster than he becomes greedy...