Why did the ideas of liberty and equality that were the banners of the American Revolution not lead to freedom for black slaves?

A few reasons—African slaves were not viewed as people in the way that whites were. Slave owners viewed it as a duty to own slaves, so that the slaves would learn Christianity and proper farming techniques. They viewed their slaves as misguided children who would always need guidance. This paternalistic attitude did not just extend to slaves, it also extended to women, Indians, and even the poor, who were considered unworthy of holding office by many of the Founding Fathers.
The slave owner also had his own economic reason to have slaves: slaves were the only way to make a large profit, which sustained the high standard of living for the landed elite. While this would mainly be an argument of the post-Revolutionary generation, the slaveholding elites claimed that their slaves helped the economies of the Northeastern and European mill towns because slaves helped create a cheap source of cotton. The slave owner thus justified his action by claiming that his role was important in the greater national and world economy.
The final reason was fear of slave revolt. There were a few slave revolts in the early American period, and they were all put down gruesomely. The Founding Fathers knew of the slave revolt which happened in Haiti after the American Revolution, and they did not want a race war in the United States. The United States did not even acknowledge the existence of a free Haiti until 1862, when Southern planters left Congress. Slavery was abolished in the North because the economy was not as dependent on slavery, but in some Southern locales, slaves outnumbered whites. South Carolina, one state where this was true, had some of the harshest slave codes in the country in the period after the Revolution.