I’d say there are a couple of reasons for Buchanan’s on-air ubiquity. First, he’s good at the television medium. Second, his dissident conservative status is a product of debates since the end of the Cold War. Those with longer memories remember Buchanan as a red-meat guy during a time when relatively few conservatives had access to the mainstream media.
As far as constituency is concerned, in the 1996 presidential race Pat Buchanan turned out to have a much bigger fan base than Phil Gramm, the favorite of conservative elites. The 2000 race, of course, was another matter.