Ok, so I might miss a whole lot in this answer but I will give it a shot.

After the Revolution, there was a hangover period (so to speak) where Americans were hopeful slavery was on its way out. We can probably call this a “freedom” hangover, although the term used at the time was “liberty”. It was a popular assumption throughout the north and south that slavery would die a natural death. 1807 is still within that hangover period. Northern states had started to abolish slavery at this time. The British had abolished slavery and would soon use their navy in the famous anti-slave trade patrols of the Atlantic Ocean. Even George Washington in his will freed his slaves, although only upon his death in 1799. Most importantly, the Constitution provided that the slave trade be abolished 20 years after its ratification. 1807 would be right on time to fulfill this promise. The hangover was still in the air by then.

The south was more complex because slavery was much more widespread. Evidence shows that, by that time, slavery was not a profitable labor model. It left slave owners on the hook for child and elder care for their human chattel, not to mention room and board. It was out of step with the capitalist economy forming throughout the Atlantic world at the time. Capitalism and the Industrial Revolution stressed efficiency and profitability. American-style slavery was none of these things. It was also barbaric and fundamentally undermined America’s claim to be an “Empire of Liberty”, to steal a Thomas Jefferson phrase. Most southern slave owners would be aware of these things at the time.

However, 1807 was also the start of a new development: the cotton gin. The cotton gin allowed for the swift cleaning and production of cotton. It just so happened that Britain, with her newly built textile factories, was in need of lots of cotton. This new demand for cotton represented an opportunity for the plantation system. This opportunity was so lucrative that it promised to make slavery profitable, since intense manpower was needed to farm and harvest cotton. It should be pointed out that the south had specialized in tobacco, rice and indigo before the rise of King Cotton. Tobacco, rice and indigo would not make slavery profitable the way cotton would with the gin.

So by 1807, we are at a crossroads of sorts. The Revolutionary generation was still alive and largely felt a duty to fulfill the promise of the Constitution by ending the slave trade. Unfortunately, the Revolutionary generation did not want to tackle the issue of abolishing slavery head-on. The overall sense was “we will take this first step in abolishing slavery and future generations will take the next steps.” It set an unfortunate pattern of kicking the can down the road on the issue of slavery. We can see this in Washington’s manumission of his slaves. Rather than face losing his slaves, he put it off to after he was dead.

And they did not want to abolish it because southern slave owners largely saw their slaves as status symbols. Sure, it was unprofitable but it was how southern aristocrats measured power. Nobody with any power in the U.S. was willing to ruin a fragile independence by driving the south out of the union by trying to abolish slavery. Better to abolish the slave trade and let future generations come to their moral senses.

Unfortunately, that did not happen. The cotton gin would soon make slavery profitable. Slave owners became the wealthiest class of people by the time of the Civil War. Their wealth was built on the backs of enslaved human beings.

Without a steady stream of slaves from Africa, Americans turned into slave traders. The internal slave trade boomed during the early 1800s. Virginia became the center of the domestic slave trade. Slaves would be bought from Virginia markets and sent to various areas around the south. Virginia would become the center or the domestic slave trade. It was the biggest slave state and around half its population was enslaved. It was the perfect place for slave owners to breed slaves for sale elsewhere. For a fuller context, I should point out that slave owners preferred domestic slaves to African slaves. Enslaved people from Africa were defiant and caused rebellions. Enslaved people who were born into it in America were seen as more docile. Indeed, well before the slave trade was abolished, southerners preferred domestic slaves. Here you can see a big reason why southerners had little problem abolishing the slave trade.

The great moral awakening the Founding Fathers hoped America would have over slavery never came to pass. The next generations would become more polarized over the issue. Look at the generation that immediately followed the Founders. It was the Age of Jackson and John Quincy Adams. Jackson was an unapologetic white supremacist and slave owner. Adams was morally against slavery and the dispossession of Native Americans to clear land for cotton (Trail of Tears). Slavery had become profitable on top of a status symbol, entrenching it and the racism on which it depended deeper in the American fabric. The hopeful age of liberty never came about and would curdle into acrimony that came to a head at Fort Sumter in 1861.

Thomas Jefferson himself had a sense of gloom towards the end of his life. He was one of the few Founders to live well into the 1820s where this acrimony was starting to gain steam. Shortly after the Missouri Compromise in 1820 (which represents another example of the U.S kicking the slavery can down the road), Jefferson said “slavery is like having a wolf by the ears. You can’t hold onto it forever but, as soon as you let it go, it will bite you.” (Quote from memory. May not be exact wording).