In 1971, the Husker allowed more than 10 points only three times, allowed more than 20 points only once (Oklahoma in the game of the century), and had three shutouts. TO inherited one of the best defenses in the history of college football. What happened? TO's weakness until the mid 90s was mediocre defense. How did the defense degrade so quickly under TO?
You can't compare scoring from the 70s to todays game and draw any conclusions. You need to compare that team to other teams from that generation. I tired finding CFB defensive stats from 1971, but none exist in aggregate.
For example, the Alabama team we beat in the Orange Bowl that year only gave up more than 10 points 3 times and more than 20 once (to Nebraska), same as NU. Not sure they are "one of the best defenses in the history of college football".