Well it is a whole lot easier to plug the couple of running lanes hat are right in front of the LBs than to go plug the ones outside. And he shere size of the 5,6,7&8 holes means that you will need more defenders to plug those holes.
Why?
Why not?
Oline can't open the holes up the middle?
Hmmm... Let's try the ouside lanes, then? How much worse could it be?