Because in stepping down he proved that a democratic republic, not really a thing yet, was possible. And ushering that level of democracy in the last 1800s, especially after the first try didn’t work gets someone a get out of slave ownership card.
Washington made it a condition of his will that the slaves he owned would not be inherited by his heir but would instead be freed after Martha died. She figured that was incentive for one of them to do her in, so she freed them at the end of the following year. Neither George nor Martha could by law free the 'dower' slaves she received through her first marriage to a member of the Custis family, so those people became the property of other Custis family members after Martha died.
It was also a stipulation of Washington's will that the former slaves too old or too ill to work would be supported by the Mount Vernon estate in perpetuity.
Not even close to what we nowadays think of as being dedicated to the principle that that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. But, credit where it's due: Washington at least took a step or two in that direction.
His crypt and the Mount Vernon estate are in private hands, his portrait is in the National Gallery, and his place in American history is assured. If people want to remove statues of him from public spaces because he was a slave owner, so be it. IMO that part of his life shouldn't be swept under the rug and it certainly shouldn't be glorified.
But I do agree with
Jimmy Higgins. Washington set an example for every President that followed him. He showed the world how the peaceful transfer of power is done. He should be remembered with respect for that part of his legacy.