So the real question is: why were black slaves reissued at two-fifths? A controversial issue at the 1787 Constitutional Convention was whether slaves were counted as part of the population in determining state representation in Congress or whether they were instead considered property and not considered as such for representation. Delegates from states with a large population of slaves argued that slaves should be regarded as persons in determining representation, but as property if the new government collected taxes on states on the basis of population. Delegates from states where slavery had become rare argued that slaves should be involved in taxation, but not in the provision of representation. After a controversial debate, the compromise that was finally accepted – counting “all other people” as three-fifths of their actual figures – reduced the representation of slave states compared to the original proposals, but improved it in relation to the Nordic position.  One of the encouragements for slave states to accept the compromise was its commitment to taxation in the same proportion, thereby reducing the tax burden on slave states. The three-fifths compromise was an agreement reached by delegates of the Constitutional Convention of 1787. Under the compromise, every enslaved American would be counted at three-fifths of one person for tax and representation purposes. This agreement gave the southern states more electoral power than they would have if the enslaved population had been totally ignored. And that is what the Union did in 1861-1865.
If they had relied on a 1 for the representation of each of the `other people`, the task of the Union might have been more difficult, perhaps impossible. The lifting of the compromise gave more representation to the South, since the members of the former black population, formerly enslaved, were now fully counted. Nevertheless, this population continued to be denied the full value of citizenship. The South passed laws such as “grandfather clauses” that were supposed to deprive blacks of their rights, while their population gave them more influence in Congress.