The weather is a main factor. As the north is incredibly colder than the south, the north couldn't support plant life, while the south had the soil, temperature & humidity needed to grow crops. As the south had most recources, it didn't have one: cheap labor. So, the labor came down to slaves. Slaves did the labor in the south, while the north used their money to pay workers the salary needed.
Another factor is the societies that the north and the south worked in. For the north, since they didn't have all the recources needed, they started growing into a much more advanced society with inventors and other people, including freed slaves and those who escaped. After the poeple in the north heard the slaves' stories, most became abolisionists. On the other hand, the south's conditions supported agricultural growth, which then led to labor costs. To keep their customers buyings & for the south's farmers to earn somewhat of a profit, they had to have free labor, so that's where slavery came in. Hopefully this helps you out!