Soils in the Chesapeake region were becoming less productive. Wheat was becoming important in Maryland and Virginia, which had previously relied more on tobacco. There was an expectation that slavery would eventually be abolished in those states. With more free states than slave states, it was hoped, the states further South might also rethink slavery and abolition.
The problem was that South Carolina, Georgia and the new states of Alabama and Mississippi had very good soil and climate for growing cotton, and there was great demand internationally for cotton, so slavery didn’t go away. North-South rivalries became stronger as the country moved westward and also played a role. The revolutionary spirit that had inspired the first wave of emancipation didn’t last long either.
This is a very interesting thread and I don’t mean to hijack it, but related to the comment above about cotton, this interesting map shows the change in the world cotton trade: https://www.loc.gov/resource/g3201j.ct002136/?r=-0.036,0.041,0.926,0.577,0
All of that is true moving forward, but the U.S. would’ve been a radically different country from the start had the Empire allowed Americans to abolish slavery when we wanted to abolish it out of our colonial legislatures instead of vetoing our laws.