Because people are always willing to sell their ico coins less than what's on the market, the selling power will always be greater than the buying power unless in rear circumstances.
I dint think any investor would want to sell lesser than what he bought because that is foolishness to have even invested in the first pace and I think that it is profit that usually driver majority of them to quickly dump their coin, and you know that it is first come first serve, and virtually most people that participate in ICO now wants to just do so for a short term purpose, so you see virtually all the investors that has bought at ICO price immediately go on sell the moment it enters an exchange.
It was in the past that investors used to be encouraged to stay with a project for a very long time, but they also started dumping their coin after we realized that there are some projects too that the developer will purposely dump unsold token on the market just to get rid of the projects, thereby crashing the price.