The Constitution can be seen as an effort to evade or delay dealing with the issue of slavery in the interests of, as stated, creating a "more perfect union." The unresolved status of slavery led to increasing sectional disagreements and further compromises over the next 73 years before the southern states began the secession process in 1860, which caused the Civil War.
In 1787, a major concern of many of the southern delegates to the Constitutional Convention was that an effort would be made by the anti-slavery members to enact abolition in the entire Union. This did not happen, of course. But the compromises that were made can be seen as both anti-slavery and pro-slavery, though the term slavery is never mentioned in the Constitution. For purposes of representation in Congress, all "other"—that is, "unfree"—persons were to be counted as three-fifths of the free population. And the "importation of persons" by any state would not be restricted until the year 1808.
Southern delegates congratulated themselves that the Convention had shown itself incapable of banning slavery. But the future ban on the importation of enslaved persons, as weak as it may seem, was at least a step announcing that slavery was being judged as wrong and that a gradual emancipation was planned, just as was already being enacted in the northern states at the time. In addition, the Northwest Ordinance had already excluded slavery from the Northwest Territory. This was not reversed.
The wishful thinking of the anti-slavery delegates was that slavery would soon "die a natural death" and that there was no reason to push the issue now and cause the southern states to walk out of the Convention. But all the Constitution did was delay the inevitable crisis; it was the start of a chain of compromises that sought to balance the wishes of North and South, of anti-slavery and pro-slavery. The Louisiana Purchase in 1803 added an enormous territory to the U.S. and extended the question of where slavery would be permitted. The Founders in 1787, just as in 1776, could not have anticipated this development—or that eventually the new country would be extended even further, from coast to coast, and would be populated by new settlers as quickly as it came to be. The Constitutional Convention could only accomplish as much, unfortunately, as its more progressive-thinking members thought possible in the reality of their point in history.