Much like in politics, war is usually decided by money. Who has enough money to finance the war campaign such that their soldiers can get the best equipment and means to keep up the war effort.
The North had that advantage over the South and other advantages flowed from this. With a superior economy which was based on industry and finance, the North could levy taxes and collect more money from their people than the South could.
With this money they were able to buy better guns, better equip their navy, supply food to their soldiers and transport them around on railroads as well. The economy of the North made it such that they had a resource advantage over the South and they used this to their advantage from the mid to the late stages of the war.
