The main stream media is not controlled by anybody other than its audience.
The news is just a form of entertainment that tells its audience whatever they want to hear.
They do have to present information that is somewhat factual, so their audience doesn't go to work the next day and feel stupid because everybody else is talking about some news event that they didn't hear about on the news.
But the news is always slanted toward the political leanings of the audience.
And audiences are historically very liberal leaning. Simply because most conservatives are too busy working or making furniture in the basement or building a boat in the garage. They dont' watch the news, they have better things to do.
Liberals are the ones who sit around on their keesters all day and all night listening to the latest gossip on the news, just so they'll have something to gripe about because they don't have the slightest bit of ambition to do anything else.
That's why the news is heavily liberal biased. That's the audience they have, and that's what they want to hear. That's what gets the ratings, so that's what they do.