Because they believe that Facebook prohibits bitcoin and ICos and other cryptocurrency
The new Facebook policy will not allow announcements of known digital currency exchanges or initial offers of currencies.
Is it to avoid fraud or false advertising? That's why?
Facebook is doing a very good job of them. They just want to protect the user by scam ICO projects being created every day. These projects will spend some money to advertise their projects to many people. But they did not expect that Facebook prohibits the promotion of ICOs and they would not be able to cheat people's money