Salted fish was common because there was a lack of refrigeration methods
Before fridges and other refrigeration methods, salting the meats was the best way to preserve them for long periods. This is because salt acts as a natural preservative. It keeps away fungi and bacteria while also maintaining a chill temperature. Fish were preserved this way in colonial times.
This method was used all around the world before the industrial revolution, and it is still used in many cultures to this day. Salted fish can last for months if they are stored in a cool environment. This was perfect for Americans in the North, as they had no problem keeping their food over the winter.