At least 25 million watch the main stream Nightly news every night; thats about 9 % of all people, close to 20 % of adults.
There still are people without cable, thats part of the problem. The rest of the problem is inertia related.
Just keep in mind those numbers have fallen through the floor over the last 25 years (I think roughly 2/3 of their former audience has abandoned them during that time frame), and continue to drop with each passing year. CBS, in particular, is getting pounded, and lost 5 percent of its audience just between the first half of 2002 and the first half of 2003, despite the war. (All left-wing all the time doesn't sell, apparently.)