The total IQ on the planet remains constant even as the number of people increase, I guess. Although I really wasn't expecting the average to go down this fast--consider the idealistic and rationally thorough political speeches of the 70s and 80s, and the way things went quickly downhill from there...
As someone said, it's no wonder The Matrix chose the 90s as the peak of human civilization.
The recent political speeches at the economic clubs of Detroit and Chicago were a little deeper than Dick and Jane! Also 21 Nobel Prize economists found the content to be deep enough to disagree!
EDIT: keep in mind that the expected "general" audience for the SOTU has also expanded dramatically due to technological change in between 1790 and 2018...
As someone said, it's no wonder The Matrix chose the 90s as the peak of human civilization.