positive statistics (i.e., points allowed) - until it doesn't.

For example, earlier in the season, it looked fine because even though our defense was on the field for far too many plays, we were keeping the score down (and winning). All the yards were being chewed up between the 20's. Seems fine. However, as the season rolls on and the 2-deep isn't interchangable, the starters play more than the equivalent of four games worth of downs, for every three we are actually playing, because they are on the field for an extra 15-20 plays a game. Admittedly, not a perfect comparison. But still an extra 45-60 plays at full speed over a three-game stretch is bound to increase fatigue and injuries. If you increase the number of plays, basic odds say that you increase the number of injuries.

The other big problem is that defense feeds home crowds and quiets opposing crowds. Big offensive plays get people cheering for a moment, but then an intelligent fan base quiets back down so their offense can hear the play call and snap counts. Stifling defenses that force three and outs with sure, big-hits and gang tackling get stadiums absolutely deafening and intimidating, while simultaneously silencing the opponent's crowd. Bend but don't break is self-defeating, even when it stops a score because it always struggles to help create team momentum. Yesterday, after Hall's long run, had we gone out and got a three and out on defense, I think the momentum would have swung.

And finally, once a team is behind, it's always difficult to mount a comeback with a bend but don't break defense, because it limits your own team's offensive possessions when it doesn't get off the field.

TL;DR - Too many plays on defense contributes to fatigue and injuries, negatively affects momentum, and makes a comeback difficult.