Good question. One that author S.L. Farrell tries to answer:
The skills you learn in short fiction don’t necessarily translate into equal skills for writing long fiction. The pacing is different: a short story needs to start as close to the end as possible while a novel may start much further back from the climax. The way you build a novel is often not something that you can duplicate in short fiction, as novels use a more intricate structure (and on the flip side, short stories can often use wildly experimental methods that work within the confinement of a short story, but which would get deadly tiresome to the reader in a novel). Scope is different, since short stories tend to use a microscope while a novel uses a wide-angle lens: you can tell the tale of a battle in short fiction, but you can’t give us the whole five-year long war. Setting is different: you generally have one or two setting in short fiction; in a novel you might have dozens — which means that the worldbuilding has to be much more in depth; you won’t get away with a painted backdrop in a novel. Plotting is different: short fiction tends to have a ’straight-line’ plot; a novel’s plot is generally more complex, and has the added complexity of sub-plots supporting the main plot. Characterization is even different: the character arc in short fiction will usually show the ‘top’ of the arc — that defining moment when the protagonist’s life is changed — while in a novel, the writer can show much more of the arc. Characterization is generally slower and deeper in a novel.