It is a common and calculated tactic of your ilk to conflate the original British Colonies with the original States of the United States. Slavery was foisted upon the Colonies by the British. The death knell for slavery in those Colonies began precisely with the creation of the US Constitution. The northern States began immediate steps for the eventual abolition of slavery concurrent with the creation of the Constitution. But I think you know that. The eventual ending of any importation of slaves was written into the Constitution.
Please try to be more specific with your broad brush generalizations. For example: "nine of the nine northern slave states voted to include slavery in the Constitution". Now do you understand how silly that sounds? Can you try to be more specific when you say "include slavery". Some people may get the wrong idea when people such as you and your ilk go running around spouting these tired lost cause memes.
If you check out this link, you'll see that gradual abolition actually began soon after the Declaration of Independence:
That left New York (1799) and New Jersey (1804) which had not yet started gradual abolition by the time of the constitutional convention in 1787.
As of the 1790 census, only Vermont and Massachusetts reported no slaves, and as of 1850, only New Jersey in the Northeast reported some slaves.
In the meantime, Southern slaves grew from about 700,000 in 1790 to nearly 4 million in 1860.
In the 1790 census, of 700,000 total slaves, about 40,000 lived in the North = 6%.
By 1860 of 4,000,000 total slaves, only 35 total lived outside the South = 0.001%
“Can you try to be more specific when you say “include slavery”.”
Article I, Section 2. Start with that.