The difference is that Facebook is a social networking site and is geared to sharing private information (not that Facebook has a good reputation when it comes to keeping private information private), while YouTube is a public broadcasting platform and should never be used for anything private. After all, if YouTube implemented your suggestion, and you blocked me from your channel, I'd just log out and access your channel anonymously. If YouTube made channels inaccessible to anyone not signed in to their YouTube account, that would defeat the object of YouTube and I would get around that by creating a new channel with a clean sheet.