Generally speaking, imperialism needed "land" and "trade" in order to work, since the land was necessary to harvest new crops and/or raw materials, while trade was needed to sell those goods for profit.
A) The South didn't need to conquer the North to win its independence.
The South was just planning on defending itself till the North grew tired, but the North had to conquer the South to be able to reunite the Union and win the war.