There are a number of reasons that landscape and other outdoor photos may be underexposed. Perhaps you neglected to add the necessary EV compensation, or an overly bright sky biased the exposure. And remember, the light meter in your camera is calibrated to deliver a proper exposure with “normal” subjects; i.e. those that are comparable to 18% gray. So if your shot contains a lot of bright snow, that white fluffy stuff will be rendered as a dirty gray.