In the mid-19th century America newspapers fell along party lines. The same speech by Lincoln might be reported differently in a Republican newspaper than in a Democratic newspaper. But by the 1970s people across the political spectrum watched, listened to and read much the same sources. This implies that the 19th century newspapers at some point became or were replaced by more neutral news sources. So what exactly happened, and at what point?
I'm not an expert in American media, I'm afraid, but this is a process that's definitely not unique to America, so I'll share some general thoughts.
First and foremost, I'll object to the idea of politically neutral journalism. It simply doesn't exist. Newspapers might become less explicitly partisan and might sever any ties they have to a certain political party, but all journalism is inherently political because humans are inherently political. Journalism, like history, is a very human process - where any and all "facts" are interpreted and filtered through human actors with their own political world-view and frame of reference.
Now, what happened between the mid-19th century and the 1970s in the newspaper industry is plainly capitalism at work, and certain shifts within capitalism during that time. One thing to remember is that newspapers aren't cheap. You need a lot of capital to get a newspaper up and running and that meant that a lot of early newspapers were either founded by wealthy individuals or by large organisations - and while there were some that were in it from a business standpoint, most had a very clear ideological drive to owning a newspaper. But regardless of motivation of the owner, almost all newspapers at that point reflected very closely the political viewpoints and desires of their owners. That's what those owners paid for, after all. Now, at the time, this was perfectly acceptable. The deontology of journalism was a lot more loose back then and the idea of detached neutrality (or at least the facade of detached neutrality) wasn't ingrained.
Now, as I said, publishing newspapers is expensive. Yet the price people are willing to pay for a paper was limited - people want to be informed, but they won't forgo a meal to read about what's happening in city hall. So here's the problem: the price a newspaper is sold at is - and has always been - lower than the production cost of said newspaper. That's a problem. The solution to that problem is simple: advertising. Advertising makes up the shortfall and ensures profits. A healthy newspapers will have a 1/3 advertising - 2/3 subscription income split. In reality, a lot of newspapers have this split inverted. Now here's the problem with that: the advertising industry is one of the most sensitive industries when it comes to economic crises. When the economy goes into a recession or a depression, companies first cut back on their advertising costs. And this directly hits the newspaper industry - it's directly tied to a volatile industry and thus volatile itself. So capitalism inevitably goes into crisis every few years and newspapers go into crisis along with it.
When newspapers go into crisis, there's generally two options: go under or get absorbed into a bigger whole. A lot of newspapers went under between the mid-19th century and the 1970s, a lot of newspapers merged with other papers and were absorbed into fewer and fewer larger press holdings and eventually media conglomerates. Those conglomerates then get absorbed into even larger conglomerates, and so on. Scale advantages protect these papers from the volatility of the newspaper industry. This is capitalism's inherent drive towards monopoly. In the press industry specifically, this is called press concentration.
Now, what does this mean for the content of the newspapers themselves? Well, for one, as newspapers merge, they reach a larger audience. In fact, reaching a larger audience becomes essential. Local municipal newspapers get absorbed into regional newspapers, those newspapers try to expand their market audience. You no longer have multiple newspapers in every town, no more ethnic press, the idea of tailoring a newspaper to a specific group becomes less and less viable. (the wealthy being an exception - there's still a niche market for the luxury press) And yes, that also means that you can't just reject half of the electorate by catering to one of both parties. It results in papers that aim for a broad appeal. This isn't a shift in ideology on behalf of the press ownership, it's an economic necessity.
There's more to be said. For instance, the rise of press agencies and the increased reliance of newsrooms on these agencies as newsrooms themselves are weakened by endless budget cuts is also interesting in this regard - the difference between newspapers shrinks and they all become more alike, as they rely on the same press agencies to supply them with much of their news. This means that it's not that people from across the spectrum are necessarily more interested in the same newspapers - it's that the supply is limited and the difference between the choices is growing less and less noticeable. There's also an interesting debate to be had about the ideology of journalism and it's evolution, but I won't be getting into that now. There's also some specifics about why this process got its dial set to 11 in the post-WW2 period, but this post is long enough already.
To sum up somewhat, the difference between the mid-19th century and the 1970s onwards (although that shift had begun earlier than the 70s) is that press concentration led to the only viable business model in the newspaper industry banking on broad appeal, which generally meant ditching explicit partisanship.