With a little experience, the beginner deep sky observer soon becomes aware that the visibility of such objects cannot be determined by published magnitude alone.
Experience proves otherwise.
Although its published magnitude indicates that this object is bright, it is in fact one of the more difficult deep sky objects, comparable to the Owl Nebula, requiring the best of observing conditions and perhaps a nebula filter to render it visible. The answer is in the size of the object. The published magnitudes of deep sky objects indicate integrated brightness, or the total brightness of the object squeezed down to a point. The reason, therefore, that this nebula appears so dim, is that its available light is spread over a large area. Although many other readily visible objects might be fainter, they are generally s maller, so their total available light is more concentrated resulting in a higher surface brightness.