I'm not sure if you really mean anthologies (stories by multiple authors) or collections (stories by one author), but either way, they rarely sell as well as novels.
Anthologies are usually agreed between the editor and publisher before seeking out stories. It's unusual to accept stories when you have no idea who (if anyone) will publish it. Known editors and big name authors are common ways to help sell an anthology to readers.
Collections tend to happen when the publisher approaches the author, due to the author having some success in short fiction publishing. It's unusual for an unknown to get a collection published (not to say it doesn't happen, but it's not common).
Basically, if there isn't some sort of marketing hook, it's unlikely you'll sell it.