The real "money" is always made by the middleman. The person who buys a product at wholesale and sales it for what the market will sustain. In other words the slave trader. In politics with money comes power, that's as true in 1788 as it is today.
In 1788 New York city was the Capital of the slave trade. This is a quote by the New York Historical Society, not the Virginia Historical Society. The runaway slave problem in 1788 was not with slaves in Virginia running away. Where were they going to run to? The runaway slave problem in 1788 was with slaves in New York running away to Massachusetts and Connecticutt, which had already passed emancipation laws, or being stolen off of the New York docks and being sold. And remember this document was being written in New York City, not Richmond.
This was the political force behind the inclusion of Section 2, Paragraph 2, of Article 4 in the Constitution in 1788, and not the southern states. In 1790 the population of the entire United States was only 3,000,000. And other than the few large plantation owners in Virginia and South Carolina most of the money was held by the Northeastern states. mainly New York and Boston cities.
The Constitution could have been ratified and a "new" United States been formed, without any of the 4 southern states, which was later to become a part of the Confederacy, being a part of it. And maybe they would have been smart not to have become a part of the United States and formed their own country then and there. But I ask you, would there have been a United States if New York had not ratified the "New Constitution"?
I personally doubt it.
As Abraham Lincoln would later state 'a house divided will not long stand'. So a country with the New England states on one side of New York, and with Pennsylvania and the rest on the other side of New York and being divided by New York, had that state NOT joined in with this "New Constitution", would not have lasted long. And if you will notice New York state thought long and hard about going it alone and NOT becoming a part of this "new" country. New York had the largest harbor, the greatest trading center and the most moneies, because of their slave trading, of the thirteen states, and would have probably been very able to have made it on their own as an independent state.
So New York state and by extention New York City, held all the cards in this game as to whether there was going to be a "United States" or not in 1788.
Now you are correct that by 1860 the dynamics of the slave population had shifted to the southern states. And the south then, at that time had a greater invested interest. BUT, that was not the case in 1788 when slavery was embodied within the Constitution of the United States as being a legal property rights issue. Everybody wants to transfer the politics of 1860 to 1788 events. That is not correct. 1788 politics were a much different situation than the politics of 1860.
Sure the southern states didn't oppose the inclusion of this clause within the Constitution. Why should they?
But, the framers of that document were surely more interested in getting New York to ratify this document than they were the southern states. Otherwise why didn't they require all 13 of the states to sign rather than just the 9 that were required before the Constitution took effect? Truth is that they did not expect everyone to sign is why. And some states were considered expendable to the new Union, and some were not.
Which of the 13 states, in your opinion, would have been the States that were considered absolutely essential to the success of the "New" nation?