Admittedly I could be way off the mark here, but the few dramas that I've ever bothered to look at credits for usually have a core/regular team in place.
If you look at the
"List of 24 episodes" wiki for example you'll notice the same handful of directors and writers throughout most of the series. Jon Cassar directed 60 episodes, Brad Turner directed 46 episodes, etc. As a big fan of Lost I remember seeing Jack Bender's name a TON as director. Prison Break's first season had 7 writers do 22 episodes. Game of Thrones first season has 4 directors and 4 writers. Frank Darabont wrote half of The Walking Dead's first season. Breaking Bad's first season had 4 of the 7 episodes written by Vince Gilligan. Banshee's first season had only two writers (and that's the best show of 2013 IMO).
Maybe those shows are an anomaly, I dunno. I just think the writing/directing on Agents of SHIELD sucks and was trying to come up with an explanation for it.