How long ago did that start? Like was it already that way even before Current Year, in the '00s?
Pretty much always been, but truly speaking the 90s-00s were definitely when news was falling into Democrat’s palms. A lot of news networks were bought out by higher entities, like CNN in being bought by Warner in 96. The more Dems captured the 6 leading big players (Disney, Warner Bros, Paramount, Universal, Sony and Fox) the easier it was to take over news.
Programming like The Daily Show likely also played a part. What little Republican spin the media had in the 00s had Stewart constantly breathing down their neck convincing America they are all fake. In a weird sense Late Night became how people got the news to the point that Stewart and co were elevated to the same status of actual reporters. It made it super easy for Obama to squeeze his way in when they rolled out the red carpet for him.
Bush is another key factor. Following above, the news became more hated for supporting him and his War on Terror. The increased pressure made flipping Dem a no-brainer from a business standpoint, the culture was pushing to head that direction.
Finally, political extremism just got upped for increased views. For a good while, Fox was the most popular for being a hard right channel, so the others went hard counter to up viewership and engagement.