You have to remember that in the north they didn't have farms or any real use for slaves the population in the north did not include many slaves.However,in the south they used slaves for their farm work.Almost every person who lived in the south owned slaves.So,the delegates from the southern states wanted the slaves to count as their population so they could get more members in the House of Representatives.
I hope this helps you didn't give any answers for me to chose from.