After learning a bit about American history and studying the declaration of independence I'm amazed at the genius and nobility of the early American leaders. However I'm taken back at the fact that it took America another 200 years before the civil rights movement kicked in and racial equality became a fundamental legal premise of the nation. How is it that the great leaders of early America did not recognize the self evident truth that just because two people have different skin tones, one man doesn't qualify as being superior to the other by the color of his skin? Personally I'm not convinced by the argument that simply because of the time they lived in, all of a sudden it's okay for them to have ignored/rejected such a fundamental principle of human dignity i.e cultural relativism
First real attempt to answer something here, so hopefully this goes over well.
There are a few key things that you need to understand about the Philadelphia Convention for this seeming hypocrisy to make sense. First off, the Founding Fathers, as great as they were, are probably given a bit too much credit for the writing of the Constitution; the document was actually written up by a group of 55 people, of which only a handful would go down in history as Founders. These 55 people represented the 12 states (Rhode Island chose not to send any delegates, for unrelated reasons) and their interests in writing a document that would govern the nation. Importantly, 25 of these delegates owned slaves, and although the level of bias this gave them on the issue of slavery varied, it played a significant part in the drafting of the Constitution.
Another key thing is that most Northern states had either already, or would shortly after the Convention, abolished slavery. In the South, slavery was still a vital institution to everyday life. As such, the debate over what the Constitution should do about slavery, if anything. was a heated one; many Southern states threatened to not ratify the constitution if slavery was banned, and many Northerners didn't want to risk returning to the Articles of Confederation over the issue, but were still vehemently opposed to wholeheartedly endorsing slavery.
The end result of the debates was the three-fifths compromise as well as various provisions against making slavery a legitimate institution across the nation. First of these provisions was that the government would have the power to ban the international slave trade, but only after 20 years, allowing the South essentially a grace period in which to prepare, while ensuring that the expansion of slavery would be much more difficult due to the difficulty in procuring new slaves for new settlements. Second, the three-fifths compromise stated that slaves would only count as 3/5 of a person in terms of representation, conceding to the South the ability to have roughly equal representation in the House of Representatives to the North, even though the aforementioned ban of trade would limit the amount of states having slavery.
While neither side was truly happy with the outcome, it showed the legal aptitude of the members of the Convention, as the compromises on slavery in the original Constitution proved far more enduring than any future attempts at averting the Civil War.
TL;DR: The Northern representatives wanted to ban it, but the Southern delegates wouldn't sign if it was, forcing a compromise.
Sources:
The Constitution
Slavery and the Making of America, James Oliver
My High School US History Class