Part of that is that every Big 12 teams plays both Texas and Oklahoma every year (whereas PAC12 North teams don't play both LA schools yearly), but the main part of it is that Texas and Oklahoma have been substantially bigger viewership draws than USC or UCLA.

An interesting example is Baylor.

If you look at their average viewership among widely available P5 games, they get 1.83M
But if you take out Texas and Oklahoma games, it drops to 1.50M (an 18% drop).
If you leave Texas and Oklahoma games in, but take out postseason play it drops to 1.12M (a 33% drop from the original)
And if you don't include postseason play AND don't include Oklahoma/Texas games? It's all the way down at 730k (over 60% dropoff)

With the first number (1.83M), they're the #5 draw of the 19 teams left in the Big 12 and PAC12 that have a reasonable sample size in P5 games (basically not including Cincy, UCF, Houston)
With the last number (730k) they're #18/19 in the same grouping, even when those same constraints are applied to every team (also USC/UCLA games removed).

So is Baylor a top draw? Or an extremely weak draw? Numbers could easily say either is true.