It was so easy to get people outraged and rage reading, that it skewed news’ business models.
Murdoch was successful in many markets before starting Fox in the US. His formula was to go after the tabloid market with higher quality content (plenty of stories on scandals and beautiful/famous people, some conspiracy theories, and also quality content). Conservative media had been dominating talk radio and there was plenty of conservative media (Heritage Foundation, National Review) for conservative viewpoints before Fox. CNN (via Ted Turner) had shown that cable news could be profitable and Fox/Talk Radio have shown that rage media creates a loyal audience.
It's not that mainstream media is liberal media as often claimed (I find much of establishment media: CNN, MSNBC, USA Today, People, etc. too superficial to be liberal or conservative -- it's just a business that tries to attract the establishment audience which tends to skew liberal). The real issue in my mind is that mainstream media has not been very good at rage media while Fox and Talk Radio have been virtuoso at it. Trump temporarily made it easy for CNN and MSNBC to thrive at rage media (all liberals could agree on their shock about Trump's actions), but without Trump, liberals naturally fall into infighting between progressives and establishment viewpoints (for example, Bernie Sanders versus Hillary Clinton or AOC versus James Carville)
Talk radio and Fox news are highly popular. As I understand it, before he was fired, Tucker Carlson was the most watched news-related show.
I agree that there are popular progressive and left-leaning shows that focus on outrage (John Oliver, Jon Stewart, etc.) but as I understand it, they are not as successful or popular as Fox News and conservative talk radio.
My point was not that conservatives are more outraged than liberals (I suspect that each side sincerely believes that the other sides shows greater outrage). I just meant from a business perspective, it appears to me that the establishment cable news services are struggling to maintain audience loyalty which tells me that they are not as good as conservative media at taking advantage of audience outrage.