Explain why slaves were needed in the southern states but not needed in the northern states.

Interesting question!

It is a huge misconception that there were no slaves in the north. Slavery was not as common due to the fact that the Northern economy was not as heavily agricultural as in the South. New York and Connecticut had large slave populations.  In the North they have cold winters and the South had mild winters. Meaning you would have to provide warmer cloths, food and warmer shelter which would cost slave owners in the North more money