Because they anticipated later the ability to use the power of the collective government to destroy it anyways. Some of them as much as said so here.
And you say this does not constitute a bait and switch?
Though I wonder if they would have told Georgia and South Carolina to go pound sand maybe they would have eventually agreed to join the Unites States and we could have ended slavery much sooner.
If they prospered, then they would not have been alone for very long. The fear at the time was that England was powerful enough to defeat them, and that only by hanging together could they create a sufficient deterrence force to convince England not to try.
Had Georgia and South Carolina rejoined England, that would have wrecked the rest of the system, because then England would have had a better means of reconquering the Colonies.
With this in mind, it looks as if the very existence of the USA as an independent nation depended heavily on convincing the states to band together.
Also my point in referencing the constitutional convention is to show that slavery was an issue even at the founding of our new government. Even Thomas Jefferson realized that slavery could be the rock that would tear the ship of state asunder.
Don't know if you have ever visited the Jefferson Memorial in D.C. but there is a quote on there of his. But it is only a fragment from a much larger passage. Here is the passage in its entirety;
The whole commerce between master and slave is a perpetual exercise of the most boisterous passions, the most unremitting despotism on the one part, and degrading submissions on the other. Our children see this, and learn to imitate it. …The parent storms, the child looks on, catches the lineaments of wrath, puts on the same airs in the circle of smaller slaves, gives a loose to his worst passions, and thus nursed, educated, and daily exercised in tyranny, cannot but be stamped by it with odious peculiarities. The man must be a prodigy who can retain his manners and morals undepraved by such circumstances … if a slave can have a country in this world, it must be any other in preference to that in which he is to be born to live and labor for another or entail his own miserable condition on the endless generations proceeding from him … Indeed, I tremble for my country when I reflect that God is just: that his justice cannot sleep forever.