When the Northern and Southern colonies met at the Constitutional Convention of 1787, they disagreed on many things, the most important of which was the control over trade. The North wanted the Congress to be able to control trade and taxes, whereas the South disagreed because they feared the Congress would then abolish slavery and thus destroy its economy which was based on free work.
Therefore, they made a compromise, known as The Commerce/Slave Trade Compromise which allowed the South to retain slaves until 1808, but also to have to pay taxes for them if they import new ones.
