A combination of schools from high population states in different regions of the country and a super-close game with last second heroics. All the elements to make for a high rating.

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2014-15 CFP: Ohio State def. Oregon – 18.6 rating, 34.148 million viewers

No real reason for this to be this high except for this:
It was the first CFP finale. The game wasn't close, Oregon doesn't bring a huge fan base.

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2009-10 BCS: Alabama def. Texas – 17.2 rating, 30.776 million viewers

Two big name programs, but more so was the fact that Texas started a come back before Alabama put it away - this brought in a lot of viewers who were otherwise just watching the score.

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2017-18 CFP: Alabama def. Georgia – 15.6 rating, 28.443 million viewers

The second highest CFP finale, despite both being from the same region. This was the most thrilling set of playoff games, and the only finale to go into OT. A very thrilling overtime led to a huge following.

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2012-13 BCS: Alabama def. Notre Dame – 15.1 rating, 26.38 million viewers

Two big names, but a drop off in viewership as most fans outside of the two programs already knew it wasn't going to be close.

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2003-04 BCS: LSU def. Oklahoma – 14.8 rating, 23.937 million viewers
2007-08 BCS: LSU def. Ohio State – 14.4 rating, 23.069 million viewers
2019-20 CFP: LSU def. Clemson – 14.3 rating, 25.59 million viewers

Thanks to a growing amount of potential viewers, the LSU game with the most viewers has the lowest rating.

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2011-12 BCS: Alabama def. LSU – 14.0 rating, 24.214 million viewers

A rematch of a defensive slugfest was probably not terribly appealing to non-fans of the two programs.

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2018-19 CFP: Clemson def. Alabama – 13.8 rating, 25.28 million viewers

Yeah, another big blow out made the game unattractive to anyone who either wasn't a Clemson fan or Alabama hater.

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2004-05 BCS: USC def. Oklahoma – 13.7 rating, 21.419 million viewers

Oklahoma's one win in this list is the third highest rated game; but they do show up on the loser column and awful lot. (Oklahoma is 1-3 in BCS title games and 0-5 in playoff games)