Very often, or perhaps more often, and even in very good collections - even in some of the best collections ever written, I would argue - it's because our "voicier" writers hew so closely to one given set of dictional tics that we as readers can't read the books all the way through in a single sitting, because if we did, the stories and their narrators would all start to bleed together.
Roy KeseySome writers keep a tighter rein on that than others. For short story collections I'm definitely in the loose-rein camp.
Roy KeseyYou know how some people say that they can only read one story a day from this or that collection? If that's simply the result of the great tension and power condensed into each piece, all well and good.
Roy KeseyI start every first draft with voice rather than theme or image or even character as such, so it isn't like I'm ever rubbing my hands, cackling, "The dad is really going to take it on the chin in this one!" Not in terms of any given story, and certainly not in terms of the collection as a whole.
Roy Kesey