It is because after token listing on exchanges bounty hunters and impatient people are selling like madness and there are no buyers because they bought tokens during ICO.
I totally agree, too many bountyhunters aiming for a quick win. What most of the people do not realize is that investment is a marathon and no sprint.
I personally have seen many scam projects fade away during summer, some even unnoticed by other ones. It´s a bad situation that gives bad reputation to the whole market.