Nielsen started measuring the ratings in bars, hotel lobbies, gyms, etc. (out of home) in April 2017. I can still hear the old CBS basketball announcer Billy Packer talking about how popular the NCAA tournament really was because it packed the sports bars, but they weren't counted in the viewership totals back in the day, and ratings were really good instead of really great.

I'd wager that the loss of viewers for the CFP game on Monday night (like all sports in 2020) was made up of 10-20% lack of bar viewers, 30% from the teams and broadcast specifics (date/time) and 50% from covid-politics-society news. Can't prove it at all, but just a hunch.