Darkness spawns anywhere + any newb who doesn't know what a Bloodmoon or darkness monster is = increased chances of players quitting the game.
I had a suggestion awhile ago related somehow to this in that witches can build anti-churches in darkness to expand darkness to the light during bloodmoons. Of course this means it will take some time for darkness to ever reach providence but it would be a better idea than just "Here spawn Darkness monsters everywhere" and then also people chopping trees or something during bloodmoon means lumberlords can ravage all the newbies who have little knowledge of this.