One reason is that slaves in the era of the Revolution were not as valuable as slaves were after 1800.
While slaves were important in tobacco, sugar, rice and indigo plantations during the Revolution period, it was King Cotton that became the economic engine of the Southern Slave Economy.
From 1793 when the invention of the cotton gin made the production of cotton economical, the industry just grew and grew as the combination of the comfortable and durable cotton fibre, with automated manufacture in the mills of industrial Britain, made a popular fabric at a popular price. Demand expanded rapidly. Supply grew to keep up. Cotton plantations expanded westward across the South.
By 1850, about 64 percent of slaves lived on cotton plantations; 12 percent raised tobacco, 5 percent sugar, 4 percent rice (the other 15% lived in towns or on smaller (usually, although some could be quite large, such as George Washington's Mt. Vernon) general farms).
As 64% of slaves were on cotton plantations, it is clear that if cotton had not become such an important crop, there may never have been a demand for so many slaves. Either there would not have been so many slaves, or the price of slaves would have been much lower. So, without cotton the whole slavery problem would have been easier to deal with.
Whether slavery would have been abolished more peacefully without the economic boost of the cotton industry is impossible to say, but the odds would have improved.