"Mainstream" is basically all of the old "dinosaur" media outlets that slant heavily liberal. A lot of people still get their news from ABC, NBC, CBS, NYT, WaPo, CNN, MSNBC, etc. As you point out, there are a lot of other sites online now, but the old outlets don't want people getting their news from anyone but them. This is why they started the whole "fake news" talking point recently to try to discredit other online news, even though they are the ones that have been discredited.