I suspect the Thursday night deal is the bigger part of it.Notre Dame is a big draw but Penn State is not. I think the fact that it was played on a Thursday night didn't help. That's a horrible time to schedule a playoff game. I guess we'd have to see how Texas-Ohio State did Friday evening to compare.
Penn State is not the draw that say, Alabama is, that's true. But they are big enough. Certainly Notre Dame is. Or should be.
This is what happens when college football tries to encroach upon the territory the NFL has owned for many decades. You can't stage the games on Saturdays in late December/early January or the ratings would likely be even worse.