The America that was formed in 1776 was much different from the America of today that is so much more regulated by the Federal government. The government that was formed under the Articles of Confederation that were ratified in 1781 had nothing in them about slavery. For, the power to regulate slavery,as with most powers, was left to the individual states because the new Americans distrusted a powerful central government after their oppression under the British.
Then, in May 1787 when the delegates from 12 states met in Philadelphia (Rhode Island send no one), the Constitution was revised to have three branches of government and to allow the House of Representatives to be represented by population. With the great number of slaves in the South, the Southern delegates wanted these slaves to be counted for representation despite their having no vote although their numbers would increase taxes for the states. The other delegates were not eager about this proposal; however, they did want the added tax revenue. So, a compromise was made and each slave counted as three-fifths of a person.
Although slave trade was to be done away with by 1800, when three of the Southern states objected, the date was extended until 1808 in another compromise. Then with this extension, many more slaves entered the country and the Southern states continued their powerful representation and 12 out of the next 16 presidents were from the South. This powerful position of the Southern states and their large tax payments to the government delayed any changes that would favor slaves. Thus, it was not until after the Thirteenth Amendment was added to the Constitution that the issue of slavery was finally resolved.