The difference between 1787 and 1861 is that real politics was still possible in 1787. Slave states didn't yet believe that everything that wasn't a 100% affirmation of slavery as a positive good was a threat to their interests.
There was still a sense - in both North and South, though to varying degrees in different places - that slavery was something the new nation had to (eventually) put behind it.
The Deep South states knew that they were getting the Old Southwest (Mississippi and Alabama) for their expansion. Consequently, there were willing to let the Old Northwest (The Great Lakes states) go.
Prominent Virginians still looked forward to the eventual abolition of slavery, and it was a long way from South Carolina or Georgia to Michigan or Wisconsin. Pioneers from the Deep South would move into the adjoining territories, not make the long trek north.
For Georgia and South Carolina, maintaining the transatlantic slave trade for at least a generation was a matter of greater importance, an issue they weren't willing to compromise on.
I am aware that some Northerners also didn't want the international slave trade to end, but they weren't threatening to reject the Constitution if they didn't get their way.
:)
I am aware that some Northerners also didn't want the international slave trade to end, but they weren't threatening to reject the Constitution if they didn't get their way.
The salient aspect of this point is that if slave states could reject the constitution in 1787, why could they not do so in 1860?