The main reason is that all major U.S. newspapers are based in cities. Cities in America are in the main run by Democrats, because they are populated, by and large, with Democrats, and very often also surrounded by Democratic suburbs. And because cities are run by Democrats, and populated by not only by Democrats but, very often, by liberal, minority, and immigrant Democrats, they tend to have laws on the books that at least formally signal a desire to serve the interests of these voting groups -- their residents, let's call them.
Newspapers, which are businesses, are subject to the employment and other laws of the cities in which they are based. Because they are based in cities, and because cities are often at the forefront of progressive legislating, newspapers tend to work under employment laws and answer to regional communities that have distinctive views about what a just society looks like. Conservatives are right to call these views liberal, but it's just as important to recognize them as the product of representative democracy within defined urban spaces (see Richard Florida for more on what it is that causes cities to vote Democratic). Newspapers, like other businesses, have to follow the local laws -- such as those protecting out gay employees -- or risk getting sued. And, historically, they had to appeal to urban or urbanizing local residents if they wanted any subscribers.
Also important: Because employment at these city-based newspapers is voluntary, they tend to attract reporters who want to live in cities. The New York Times, for example, gets the Iowans who want to leave Iowa and live in Manhattan or Brooklyn. It does not get as many job applicants who want to live in traditional rural communities, because it is not a rural-community-based employer. Newspapers hire people who can deal with working in cities -- big, major, complicated, diverse, progressive cities -- and who will obey the socially progressive laws of those cities at work, even if they live off in the 'burbs somewhere.
There are successful conservative newspapers in cities, but they are usually the scrappy local underdogs to the big mainstream dailies bought by the plurality of the regional paper-buying population. Think: The Boston Herald (conservative) versus The Boston Globe. The New York Post (conservative) versus the New York Times and Daily News. The Washington Times (conservative) versus the Washington Post.At Reason, Matt Welch responds:
Well, let's take as a test the largest city in America you might describe as right-of-center: Houston. Fourth-biggest city, 12th-biggest daily, 7th city ranked on the largest-dailies list, politically mixed but a whole lotta conservatism headquartered and represented. Famously hostile to zoning, friendly to business. Are those politics reflected in the Houston Chronicle?
My exposure to the paper is very limited (and very positive, for what it's worth), but I don't recall any particularly conservate or libertarian point of view, or reputation thereof.
Like so many American dailies, including the Los Angeles Times (my former employer, and the plum property in the Tribune roster), the Chronicle was a strongly conservative newspaper as recently as the 1950s, before more a more progressive breed of journalist began gaining a foothold in the 1960s. Crucially, the transformation from right to left, from crassly political to high-mindedly "fair," went hand in hand with the paper benefiting from and engaging in newspaper consolidation. It was the classic deal between mostly liberal newsrooms and mostly conservative boardrooms: Close down the competition and use the profits to professionalize the news divisions, instilling a more liberal ethos even while embracing the advertising-friendly pose of objectivity. Then sit back and enjoy the 20 percent profit margins for four decades.