The media world in a nut shell:
1. Revenue is generated off of advertising dollars
2. Advertisements sell best when you have access
3. You gain access by building a rapport with high profile interviewees.
3a. Don't say anything too controversial or you will end that rapport
3b. Create enough controversy that you get advertising dollars.
This allows for high profile interviewees such as Trump and Clinton to get glamorous interviews while spokespersons get shafted. This is also why Hillary and Trump get these polite interviews while Sanders gets more of a hard hitting interview.
4. Don't make the our company look bad.
As some of these people get a bit more clout such as Rachel Maddow, they get the opportunity to shirk some of the rules, but for the most part, you see these people abide by the rules of the media darlings so that they can build up their reputation and get more press time inside the company.
Liberal or Conservative biases aside, all work the same.
If the Democrats started attacking campaign donations aggressively like Bernie Sanders, you would see a shift in liberal bias by the media because guess who benefits from campaign donations.