Why, if all the monsters died, does the labyrinth require one to be at least a D rank?
OK, there are probably still traps and such.
Maybe it has to do with the difficulties involved in keeping track of where you are, etc., that those skills are generally inadequate amongst those below D rank; you wouldn't want to allow someone who has no clue about mapping, etc., into a labyrinth; they'd never find their way out again.