The market is unregulated and there is a lot of people trying to get rich quick. They will fall victim's of the news FOMOing, leading them to buy the ATH's rather than the dip. Those people most likely won't be able to hold their position after the market goes into a downfall creating the "crash" effect. The same people will never return but they were in to make money, not for the technology itself. The market then resets with the true value of the coin intact. After that state we reach new ATH and the cycle keeps repeating. We are in the downwards trend right now, what do you think will happen next? ;)
My hunch is they will return once cryptocurrency becomes stable money. By then, all the future value will be priced in and fluctuations like this will be much harder due to a larger market cap and more and more people unwilling to sell for fiat.