For my opinion is because the market will be bankrupt or to lose so I think that is why? , the risk to be precise. To be honest im doing some basic search regarding this and here's what I got.

Link : https://www.google.com.ph/amp/s/www.forbes.com/sites/forbestechcouncil/2017/12/11/how-will-the-china-and-south-korea-ico-bans-impact-cryptocurrencies/amp/