One thing you've missed. Until the 1980's, with the removal of the Fairness Doctrine law and the rise of cable TV in urban areas (with more channels available), there were only a few major media outlets available to the public. The Big Three television stations, plus a couple of local ones, some radio stations which were interlinked with the TV ones, and a few major newspapers. People followed them because that's all there was. There was no Fox News thirty years ago. With this media oligopoly, the media could pretty much run whatever content they wanted.
Bang went the old system when more news outlets were readily available. The major news outlets probably never expected there was a serious market for conservative opinions. Of course, with the rise of Fox News, the old-time news outlets stuck with their old formulas, to keep their old customer base.
Also, it was the Internet that put the kibosh on Dan Rather's career:
http://en.wikipedia.org/wiki/Killian_documents
The late Robert Novak said in his column that for an experienced newsman like Dan Rather, it couldn't possibly have been an honest mistake.