Baseball fans, that’s who. Reports Nielsen:

Nielsen just issued the ratings for the third presidential debate in the 56 local markets where we measure viewing with TV meters. Among the highlights.
The overall rating was 38.3. This is lower than the second debate (which was 42.1) but higher than the first (which only had a 34.7 rating). The ratings were affected to some degree by Game Five of baseball’s N.L. Championship Series game between Philadelphia and LA, which was broadcast on Fox. (L.A. and Philadelphia are the second and fourth largest TV markets respectively.) Baltimore, Washington, DC and St. Louis had the highest ratings and New Orleans, Los Angeles and Sacramento had the lowest.