Ever since the emerging popularity of right wing talk radio and Fox, it has been advantageous for them to portray the traditional media as being the opposite, ideologically speaking, of themselves. So the term mainstream media became and epithet, a thing around which to rally support (viewership and readership) around themselves. Of course that is very self serving but also a great business model. Instead of there being just news, from then on it became "our" news, not "the" news.

I think the difference between the so called mainstream media and the alternative media today is that the former is characteristic of real journalism and the latter is by definition opinion, if not outright advocacy.

To draw this out just a bit further and to relate to what we are experiencing today in our society, the seeds of the division so prevalent today were sown when a certain portion of the populace opted about 25 years ago to being informed with a heavy dose of politicization added in.