Because the southern states would never have agreed to ratify the Constitution if it were seen as an instrument for ending slavery. There would have been no USA, just a bunch of small countries like Virginia, New York, Massachusetts, etc., all of which would have been easy prey for the European great powers. Accepting slavery allowed the states to unify, ratify the Constitution and form the nation that we know.
Furthermore it was widely believed that slavery was in decline anyway and would end naturally. That probably would have happened but for westward expansion and the invention of the cotton gin. In the late 18yh century, cotton was not a particularly profitable cash crop. Slaves worked more on tobacco, rice, and indigo plantations. Demand for these crops was mostly steady. The industrialization of the textile factories and the invention of the cotton gin changed the economic situation totally. Cotton became immensely profitable, and the cotton gin allowed unskilled slaves to process the crop easily. This led to an increase in slavery, which was exacerbated by the expansion of cotton plantations westward into Tennessee, Alabama, Mississippi, and later Texas, Arkansas, and Louisiana. The net result was that slavery, rather than dying out as the Founders thought it would, became entrenched in Southern society, so much so that only the cataclysmic events of the 1860s could end it.
“Accepting slavery allowed the states to unify, ratify the Constitution and form the nation that we know.”
Who accepted slavery?
Did those who were held in slavery voluntarily accept slavery?
Were slaves consulted to see if they accepted slavery?
The westward expansion added virtually no suitable land for slavery. What changed slavery in decline was the cotton gin. It suddenly made slave labor very profitable.
The net result was that slavery, rather than dying out as the Founders thought it would, became entrenched in Southern society, so much so that only the cataclysmic events of the 1860s could end it.
Slavery was paying for 3/4ths of the revenue provided to the US government.
“Furthermore it was widely believed that slavery was in decline anyway and would end naturally.”
That is the part they don’t teach. Many Southerners wanted to end slavery, but believed it was unacceptable to do it overnight.
Northern states had phased out slavery over many years and compensated owners for the loss in equity.
Southerners had long considered similar processes. They feared the social and economic impact of simply freeing millions of slaves.