This got me thinking about the partisan press of the 19th century. That link is an interesting read about it - the presses became so partisan because the parties were paying for the presses, pretty simple. As the popularity, and cost, of papers rose parties were less and less able to fund the papers. Fortunately, national department stores were on the rise and so they were able to pay for national presses. These department stores pushed for a more neutral tone for wider appeal and helped kill the partisan press.
What's interesting is what's pushing this new partisan press. As we are more and more able to monitor market fragments, advertisers want more and more to be able to speak directly to those segments. By creating a niche partisan press, you can create a niche audience, and sell them niche products.
Even larger sites do this, like Facebook creating blue and red feeds. Knowing which stories you follow is a strong indicator of your demographic. Advertisers will pay for that demographic information to better target you and so major organizations are incented to give you biased news - as long as that news continues to reveal your niche.
I'm really curious as to what will stop this. I think the sleeping majority that got trump elected aren't the super conservatives, but the middle moderates who wanted to "shake the system". I think that group will get annoyed, not that the truth is being suppressed, but that you can't easily get it from any one place. Once that demographic gets big enough, it'll be easy pickings for a smart news organization.