Historically, the biggest reason was probably because newspapers were centered in large cities which were predominately dominated by Democrat party machines. Nowadays, it's likely because American culture is dominated by the outlook of its largest cities, giving it a progressive outlook.
It did vary from newspaper to newspaper. The most famous one, the New York Times, was originally a conservative Democrat newspaper. They were generally loyal to the party except when they saw its candidates or policies as too radical. Therefore, they consistently opposed William Jennings Bryan, and later opposed much of FDR's New Deal, and endorsed his opponent in the 1940 election. The last time they endorsed a Republican presidential candidate was Ike in '56. It's relatively interesting to see their endorsements, which can be found here:
https://en.wikipedia.org/wiki/List_...ction_endorsements_made_by_The_New_York_Times
Meanwhile, Henry Luce was the magnate that founded Time, Life, and Sports Illustrated. He was a conservative Republican.
William Randolph Hearst, the main inspiration for Citizen Kane, was an outspoken populist progressive, and positioned himself to the left of most other newspapers. However, most of this was probably in-genuine, as he did stupid shit like shilling for Hitler.
Early TV news was relatively unbiased and bland. Most people also had high trust in the media. I'd say that it gained a somewhat more liberal edge in the 60s and 70s, when some insurgent elements became more outspoken progressives. This is when you get people such as Jane Fonda who posed for a picture on a Viet Cong anti-air gun. More open punditry followed cable news in the 90s and 2000s. The internet gave them an even greater outlet. Not just liberal ones though. Why do people forget that Tucker Carlson began his career on CNN? Modern day news outlets are probably center-left at best. Most far-left outlets are based online, and ironically become more Trump-ist the further left they are. Black Agenda Report is a great example, where despite being a socialist black outlet, they believe that the Trump-Russia collusion was made up and that Trump has a decent foreign policy.
Sorry for the ramblings.