Short films are short because they skimp on components that are required in feature-length films, or they cut them out completely.
Character Development: This is the most common omission. We often know just enough about the character to be able to tell the story. In a bad short film, we know even less about the character than we really need to. To be successful, we need to be able to strip the character down to the bare minimum of what we need to know, and then we need to be able to convey that information succinctly and rapidly. There’s no time for gradual growth, though certainly the character can learn something, or we can watch the character experience something that we assume they will learn from.
Beginning, Middle, End: They say all stories have a beginning, a middle, and an end. However, most short films tend to cut down the beginning and/or the end drastically, and often one or the other is just completely missing.
Story: Many short films simply do not have a story at all. They can be a character study, or a portrayal of a situation or conflict without really starting or resolving it. If a short story is a slice of life, a short film is frequently just a snapshot in time.
Required story elements can’t simply be omitted (many short films do, and in my opinion, they are failures as a result). You basically have two choices: compress them into minimum screen time, or make up for their lacking by making the other elements spectacular.
Perhaps that is the formula for a successful short film: choose one story element to emphasize and excel at, and then distill the rest to their bare bones.
Note that cinematography is not a story element. Making a film with great cinematography and nothing for a story will result in a film that only appeals to other cinematographers.