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?
There are a limit that they prohibit but not totally banned advertising in Facebook, Thus, they came up for that policy because Facebook wants to protect people against fraud and scammer, Many people in Facebook using it to scam and trick to get money from user of facebook.