score:10
Kentucky to the Union given,
Vermont will make the balance even;
Still Pennsylvania holds the scales,
And neither South or North prevails.
(*) A slogan appearing in a journal at the opening of Congress voiced the politics of equilibrium underlying the compromise that allowed Kentucky into the union. Source: Editorial Note: on the Admission of Vermont and Kentucky to the Union
Like most state issues in America occurring before January 31, 1865, the underlying issue was slavery. Kentucky's admittance to the union was dictated by slavery. Vermont was an independent sovereign state (like Texas) when it petitioned entry into he union. Kentucky was part of Virginia. The Virginia legislature separated Kentucky from the state in 1789 in preparation for it to be entered into the union as an independent state. Kentucky's entry into the Union is what allowed Vermont to enter into the union and visa versa. It was all about preserving the slave and free state equilibrium. That they didn't enter the union at the exact same time is irrelevant, the votes to admit them both after years of debate, occurred 10 days apart. It was a package deal.
Adding Kentucky to the union without a free state such as Vermont ready to join, or visa versa would have upset the delicate balance which defined the United States government until the Kansas Nebraska act May 30, 1854. Kansas Nebraska act irrevocably blew apart this equilibrium, condemning the country to the civil war, and one side of the slavery issue would be destined to be victorious.
Kansas Nebraska act allowed states themselves to decide if they would be slave or free. Before that congress decided; and if you were a single state you could be sure half the congress would vote against your inclusion and you likely wouldn't even get a vote on the hill.