If you look at the picture I've attached which is from ODU's last TD run, you will notice, exactly why we are giving up so many big runs up the middle.
There is nobody in the middle of the field at the second level. We played this play with 4 down linemen and two linebackers with a nickelback. ODU has come out 4 wide (two on each side) spreading our safeties out with them, both safeties are aligned outside the hashes. One of our linebackers immediately comes straight downhill but is getting blocked, the other one tries to come in from the near side around Leggs and the ODU lineman blocking him. He has a good bead on the ball carrier but misses the tackle. Now the ODU back is in the second level of our defense and there is NO ONE there.
Not only is there no one there but the safeties who are spread out wide are playing with their backs to the middle of the field (which has to be how they are coached to play it, because they do it every time). By the time they will even notice it's a run the ODU RB is 12 yards downfield at full speed, there is no chance they are going to be able to turn and make the play now.
This is completely a scheme issue. If you tell an offensive coach, that he can get a hat on a hat along the line and one on a linebacker leaving only the other linebacker and his RB 1 on 1 the offensive coach will take that matchup ALL DAY LONG. Now when one of our D-linemen (which actually does happen often) gets penetration or off the block, we often get a stop. Or when the linebacker wins the 1 on 1 matchup with the RB we get a stop. But.. If you run this enough against this alignment, you are playing with fire and at least 3-5 times a game the offense is going to win here, even if your defense is more physically gifted then they are. It's just the law of averages, and when the offense does win because of the alignment it is gonna be a BIG gainer. So, if you've got the opportunity as an offense to hit a play like that 3 outta 10 times, you'll take it.
I haven't gone back and really looked at the NC State game yet to see if it was this way on all their big runs too, but I know we were in this alignment in one of them, and just like here, the linebacker had an opportunity for a tackle and short gain, but the RB made him miss and then it was nothing but grass in front of him.