Northern newspapers usually exhibited strong political partisanship, because

Northern newspapers usually exhibited strong political partisanship, because they were usually financed by specific political parties.

I believe the answer is: they were usually financed by specific political parties.

The political parties tend to want that every information that given by northern newspapers would benefit their party in away. Because of this, they tend to be really political and the way they represent the news tend to be really bias to make the political party look better over the other.