Question

What is a reason the New England and Middle colonies had fewer slaves than the Southern colonies

Answer 1

There was more slaves in the southern colonies than in the Middle colonies and New England because they didn't think it was right to have slaves and they had mostly everything that they needed and could do it by themselves. 
And also the economy in the southern colonies was based on plantation farming. Plantations required large numbers of workers, so wealthy landowners bought lots of slaves. In the northern colonies, the geography and climate weren't favorable for large-scale farming. Up north, there wasn't as much good farmland, and the growing season was shorter. Northern farmers didn't tend to have nearly as much land to work, so they didn't need as many workers.

Answer 2

There were generally more slaves in the Southern states because the south had better land and climates for farming, but also the south's economy was based on agriculture so they need workers in the fields, thus the slaves. The North had a larger population of slaves when the English first settled in America but as the population grew and spread in different directions slavery became less popular and slavery became more frowned upon in some places in the North. The North also became a very large industrial area and slaves were not needed nearly as much. As for the middle colonies different areas in the middle colonies had more or less slaves because they either fell into the agricultural side or the industrial side.