Short stories are complete, and thus you know (nearly) right away what you are dealing with—whether you like it if not why, and whether you want more. They are self-contained, offering all you ever need know about the given situation. And yet they are by nature constrained and thus spare—non-essentials are left out, leaving space for the reader to slide inside, inserting imagination of whys and wherefores, physical descriptions and psychological profiles.
To read the rest of Rebecca's very eloquent post on why short stories will never die, click here.