Just read a great piece by Marion Zimmer Bradley about why stories get rejected. The key piece of advice is:
EDITORS DO NOT BUY STORIES BECAUSE THEY ARE WELL WRITTEN
The piece then goes on to list the myriad reasons why editors do buy stories and the even larger number of reasons that they won’t.
It’s nothing I haven’t heard before, but its put in a very succinct way that I think is worth a read for any aspiring writer: