Question

Why was the demand for slavery higher in the Deep South

Answer 1

The demand for slavery was higher in the South because large plantation owners had too much land for their family and or hired workers to harvest. Slaves worked for free, and paying for food and clothes for slaves is much cheaper than paying workers to harvest your fields. 

Answer 2

Probably because, In the deep south there was more farms and plots of land that needed tending so the people of the south bought slaves to do the work they had to do.