You have to take internet into consideration, fewer people watching television now. More people go online watching afterwards
Here’s some more information I’ve found:
“Obama’s speech garnered nearly 60 percent more viewers than John Kerry’s Dem acceptance speech four years ago.
And, as best Nielsen can tell, Al Gore drew somewhere around 22 million viewers with his acceptance speech in ‘00 (about the same as George W. Bush’s speech at the Republican convention), and Bill Clinton about 27 million-ish in ‘92.”