Most historians say that newspapers in the United States stopped being associated with one political party because it was not good business.
At first, it did not cost very much to publish a newspaper. The political parties subsidized the newspapers and the newspapers adhered to the parties’ lines. The papers had little other choice because there were not many other sources of revenue.
But then things changed. It became more expensive to publish a newspaper and the subsidies from the political parties were no longer sufficient to keep the papers afloat. In addition, the papers started to get more money from advertisers like large department stores. These advertisers wanted to reach the widest possible readership so they did not want the newspapers to be read by people from just one party. For these reasons, there were economic factors pressuring the newspapers to stop being so partisan.
So, of the choices you have given us, it is most accurate to say that the newspapers became less partisan because the publishers wanted to reach a broader audience.