Because 1. lack of liquidity - Very few are actually trading a single coin and usually holders have too much of one coin. Anything less that 1,000 active traders on a single coin will see strong volatility in altcoins. In Bitcoin, which is less volatile than altcoins, around 100,000 active traders.
Second, there is no real usage/demand for the coins, all speculative. Unlike wheat, oil, rice which can never crash so bad.
but oil ,wheat prices are regulated by the governments but the crypto is decentralized , moreover it is the fact that some addresses possess the significant price of the certain coin not all of the crypto currencies according to you particular coin should dump not the whole market