Well of course, slaves couldn't end slavery, they had no political power. It could only be those with political power who could end slavery. I would point out that Dr. Sowell was not a slave when he wrote his theses.
Alas, Dr Sowell won't appear on this forum, nor will William Wilberforce, nor will Lincoln. We have to make do with what we have. But with Wilberforce and Lincoln both, the competition of slave labor with the economic interests of common laborers was a powerful motivation to end the slave economies :>)