Then they would have had to go through the hassle of securing permission of any actors in those scenes for using their likeness in the show. You'll note when we do see flashbacks in Dead City to Negan beating Glenn to death, the clips are carefully edited so we only see Negan and Maggie (who are this show's leads and therefore don't need to be sought out for permission) and Glenn himself (clearly they did acquire Steven Yeun's permission) and no one else.but they could have done more...spliced in old footage to save a couple of bucks