Good Question. I have often wondered the same.
I think that the format of hubpages is best suited for short articles, preferable non-fiction hubs that are google friendly... i.e. content that people around the world are searching for. eg. "How to" hubs.
Google is the monster that drives traffic to your hubs. Mostly short stories and other fictional hubs are read by a few hubbers but not many actively seeks it out or search for it. Thus, fiction material is not google friendly. Also, perhaps people prefer to browse through ebooks on kindle etc. for their fictional fix.
So if you are on hubpages to make money and get as many eyeballs as possible, fiction and short stories may not be the best idea. But if you just want to write short stories because you enjoy writing them, by all means go ahead and write them. You will surely find a small but avid bunch of readers here on hubpages.