There probably was some playoff fatigue, and just fatigue in general with as long as the season went. As I wrote before the game, Ohio State was expected to win big, so I'm sure that played a factor as well. And while Notre Dame is a big brand, it strangely lacked star power.
SEC fans, by large, can't stand either program. I'm guessing the ratings drop was pretty big in the South.
Add it all up and not really a surprise.