I think there were a couple or three reasons. One is that the defense was controlling the game in a way that hasn't happened anytime recently. Sometimes 35 points is a bigger lead than at other times. Bob doesn't have the freedom to do a do over if an unexpected turnover triggers a comeback by the other side. OU controlled Akron's offense in the way that, well, Notre Dame controlled Texas. Very similar games. Texas should be better than Akron, but they had a race to decide whose offense was the worst of the week last night. OU's defense played a very tight, assignment sound game. It wasn't Florida State NC game efficient or dominant, but it was a great opening night performance. Akron wasn't going to score 35 last night if it had been a goal line drill in practice. It was a safer lead.
Two I believe has a lot to do with it. Why did Bob stop running a true spread in 2004? There were really two reasons. One was that he'd just recruited the best I-formation tailback since OJ Simpson. Okay, maybe since Herschell Walker. And AD was a lot better at tailback than he was at spread single back. But it was more than that. In 2000, 01, 02 and 03, Bob had seen his quarterbacks get maimed at sometime during the season.
Heupel couldn't throw an out route the last month of the NC season, but willed that team to a national championship anyway. 2001 the whole line got beat up, and shortly, the quarterbacks. We lost to a poor OSU team at the end of what had been a very promising season. And then with probably Bob's best defense at OU, barely beat Arkansas in the Cotton Bowl, despite the defense just toying with Matt Jones' offense.
2002, Jason White got hurt. Hybl limped home. He healed enough to be Rose Bowl MVP, but the offense wasn't as productive once the quarterback got hurt and the second quarterback was hurting.
2003, OU lost to LSU for the national championship because Jason White could barely walk. So Bob kept some spread elements in the offense, but ditched the pure spread. He got %tired of not having a quarterback close to 100% healthy for November or the bowl season.
So point two is that Bob understands the quarterback health risk, even though he'll never discuss it publicly. And the backup needs more reps.
I think three might be that the race between Mayfield and Knight might have been razor thin and hard to make. So Knight deserved some game reps. Let's not forget that this isn't really something new. In the opener, when Knight unexpectedly won the starting job, when the expected decision had been a future NFL tight end, Blake Bell played the whole fourth quarter in the first two games of the season.
The notion that this was Bob doing something different than ever before, just isn't the case.