Once the NFL playoffs start, the casuals stop caring about CFB.
On paper, ND and OSU which are both top 5 fanbases in terms of size (not in quality of human beings), and the fanbases both have "enemy" fanbases that are large too (mich, usc) which will draw hate watchers.
I think the NFL starting + CFP dragging out for over a month killed the momentum and interest. Also, despite the game getting somewhat close in the 4th, OSU was in complete control the entire game, so it's not like you had to tune in to watch a close game, shyt was essentially over by halftime, so I'm assuming that causes a big drop off in viewership.