It seems to be advertising revenue, which gives media the incentive to appeal to a broad audience and the means to be independent of particular donors and interest groups:
The source of media revenues is an important
determinant of media behavior. News coverage depends on the preferences
of those who pay the costs. In a theoretical model, I argue that higher
potential advertising revenues increase the value of news outlet’s
audience and thus decrease media dependence on subsidies of interest
groups. The model shows that higher advertising profitability implies
lower media bias and less distortion caused by the presence of special
interest groups. I use data on 19th century American newspapers to test
the model, showing that there were more independent newspapers in
counties with higher profitability of advertising. The effect of
advertising works through both the entry of new newspapers and changes
of affiliation of old newspapers.
That’s from a new paper by Maria Petrova, who is on the job market this year from Harvard.