It has way more to do with how much the game has changed. The offenses that are ran, how much throwing the ball has developed and then add into that the rule changes that greatly favor the offenses. If this was 15-20 years ago Bama would still be in these playoffs. Now that athletes are getting put in space/spread out it has piled on so much pressure for the defensive players/coaches. The hardest thing to do in athletics IMHO is covering a guy one on one and tackling in the open field. There's a reason why when the spread really hit the SEC the defenses have been looking more and more flawed.
This is why I am not one of those fire Mike Stoops guys. When you watch the Baylors, TCU's, and Oregons of the world run their offenses it's scary as shit as a defensive guy. They spread you out and go really fast so your players have little time to react and on top of that the coach in the booth is looking at your alignment and calling the play down. Bam, all within 12-15 seconds of the last play. One missed tackle or alignment and they pick up 20, 30 or a TD.
Saban's not a dummy though that's why he wanted the play clock rule implemented and why he went and got a Kiffin. He's a pro style guy but he runs an explosive offense. Saban knows that he's gonna have to out score some people.