i think this sounds like a good explanation:
Annette Gordon-Reed’s The Hemingses of Monticello brings into focus the paradox of the great Thomas Jefferson, drafter of the Declaration of Independence of the United States, a man of the Enlightenment whose whole personal life and well-being were tied emotionally to his slave family, which was the fruit of his relationship with his own slave, Sally Hemings, 30 years younger and the mulatto half-sister of his dead wife.
How did he justify himself when he kept his slave children and their mother close by him through his whole life, deriving comfort and love from them and yet depriving them of the freedoms that he vaunted for the newly born republic of the United States?
So it is the hypocrisy, or at least immutable dichotomy that we come back to. Washington, tho’ a slave holder, at least had the grace to release his slaves upon his deathbed. Nothing of the sort from Jefferson. Early in his life, he seemed more conflicted over the slavery issue, but became increasingly intransigent regarding the same as he aged.
I would have liked to have seen what Jefferson would have done had he been present as a Virginia delegate at the Constitutional Convention rather than serving as Minister to France during that summer of 1787 in Philadelphia. Reading Madison’s notes from the Convention, there seems to have been at least some semblance of willingness on the part of the southern delegates to discuss an end to slavery, such as phasing it out in stages starting in 1809, but no one pushed the issue. Jefferson’s presence – and assuming his willingness to move the issue forward had he been there – may have helped. If Jefferson hadn’t been willing to create an end, the hypocrisy of it all would have only grown with history. After five months of haggling over numerous issues, however, I’m afraid it became all too convenient for the delegates to shelve the issue so that they could return home.
That said, many of Jefferson’s writings are towering works of the Enlightenment’s effects. In one of life’s great ironies, his writings on natural rights (via Locke), liberties and self government remain a high-point in human history, and shouldn’t be discounted solely on his relationship to slavery – a single issue, but a large one I admit.
What Jefferson didn’t realize in his lifetime, and what subsequent events have made all too plain, is that his cherished American Republic vanished partially as a consequence of his inaction or refusal to decidedly change his mind towards emancipation. The American experience has since been the poorer for it.