Here’s another reason why stories are rejected, dare I say, some pretty good stories are rejected:
The story is too long.
The writer has either left in scenes/parts of scenes that don’t contribute to either plot or characterisation, or, more commonly, the writing isn’t tight enough.
Both will of course depend a fair bit on style and personal preference, but it’s fair to say that magazine editors are almost always pressed for space. In my reading of stories across various venues I’ve seen very, very few stories that are 10,000 words that are the right length. These days, my warning flag goes up for any wordcount over the 6-7000 word mark. Usually, not always, but mostly, the writing of these stories is flabby and wordy. The sentences carry lots of empty verbiage. The story might be fine, but the writing is not efficient enough.
Writers: try to cut every word that doesn’t pull its weight before you submit. This can sound pedantic, but can make a huge difference to the quality of the story.