That's kind of not true. The Constitution specifically set aside any discussion of slavery for a period of 20 years, the Framers knowing that slavery was the one issue that would kill the Republic before it had any chance to live. If any specific anti-slave-trade or anti-slavery had found its way into the Constitution, it would not have been ratified in the South. And when Philadelphia Quakers, led by Franklin in the last years of his life, tried to get the new Congress to debate the issue of slavery specifically, a deal was brokered to more or less "table" the issue for another day.
It took a real, live shooting war to decide the slavery issue. The Constitution, as written by the Framers, was purposefully silent on the issue.
Not entirely accurate.
1. The Constitution of 1787 called for the end of slave trade after the year 1808.
2. It imposed a tax on slaves imported during that interval.
3. It counted slaves to 3/5 of a person in terms of apportionment for Congressional representation.
4. In 1887, it is likely that all the slave states (There were still Northern slave states as well) with the exception of South Carolina and Georgia would have agreed to a gradual ending of slavery in the United States. Virginia and North Carolina had very active and growing abolition societies at the time and legislative pressures in all the northern slave states was becoming overwhelming.
5. The Northwest Ordnance acts of 1787 and 1788 forbade slavery in the territories north of the Ohio river.
There was pressure and sentiment across the nation at the time to end slavery. By the 1830s, events and economic realities (long fiber cotton) reversed much of that sentiment