You can only blitz if you have faith the back end can cover. If you blitz with shotty coverage then you either get home and sack the QB or you get burned badly. At least with dropping 6 or 7 you know you're not getting burned deep. You rely on one of your pass rushers to get home, maybe force Ryan to hold it too long for a coverage sack if you only rush 3. What it comes down to is that GB hadn't been pressuring Ryan consistently the entire game, so you have to play conservatively and rely on dropping more in coverage. Maybe if you had Matthews, Gunter and Rollins or Randall or Shields you can take chances, but a depleted D with Goodson and Hawkins starting young have to give them help.