Money - that's it.
What we used to know as "the news" was considered a public service and was a cost-center rather than a profit-maker for the networks. It was a cost they were willing to bear in exchange for access to the airwaves.
Now - even if the pundits on TV share your political views - the network is just gambling that enough people that think like you will tune in. "Truth" is a marketing term now instead of a journalistic goal/ideology.
No doubt there are ethical journalists out there - but even Walter Cronkite would be out of a job if his ratings slipped.