Facebook, as a business, is primarily interested in making money and would love to avoid all these messy issues regarding what to censor so that it can focus on selling ads.

Therefore, FB would love to have a legitimate external authority, e.g. the government, mandating what's allowed or not. Unfortunately, no one is willing to do that dirty work, and thus forcing Facebook to do it themselves.

(Granted, I've simplified the matter quite a bit here by saying "Facebook, as a business". Facebook is also a collection of individual employees who have their own beliefs and do shape FB in their image.)