Don’t know but it reminds me of an old “Mad” magazine article from the pre VCR days where someone interviews a TV executive. The questioner says “Is it true that there are only 6 hours of good shows combined on all three networks?” The TV executive says “Yes, which only leaves 2 good hours for people to watch since us three networks always schedule them for the same time. Can’t let the competition get a leg up on you.”
WAG but it could be after 60 years of television, TV executives know, or think they know, just when people will watch TV or not. It’s probably a homogeneous group and not likely to try something different.
Were the networks doing post-debate nonsense at the times Leno and Letterman would be broadcasting? If so then one day would have been preempted anyway.