Thomas Jefferson: We hold these truths to be self-evident, that all men are created equal…
Jefferson’s slaves: Wait, what about us?
Thomas Jefferson: not all men
Well, to be fair, introducing legislation to abolish slavery at the time would have divided the union. I mean, when Lincoln did it, he had half the country on his side at least; not so much on Jefferson’s case.
Also, he couldn’t free his own slaves because someone else would claim ownership of them before they could get very far.
So really, the best thing Jefferson and the other founding fathers could do was just make sure the men under them were well taken care of; a promise not every slave owner could upheld.