Because Newspapers then, just like now, shade the truth to favor their own party. Newspapers were not where you looked for independent thought.
Have you ever heard of "Declaration against interest"?
The legal system generally regards statements against interest as being true.
So Northern newspapers would say the same things that Southern political leaders and Southern Newspapers and Foreign newspapers said about the North receiving vast amounts of tariff money from the Southern states and getting vast amounts of business servicing exports from the South because..... well....... because why exactly? Why would they tell their own readers in the North that the North was highly dependent on Southern exports?
The bottom line is they had no reason to lie about this.