By the time the Revolution ended, Washington was already a God in the public view. He could have made the decision to free his slaves then, but he didn't.
It most likely didnt cross his mind. At the time it was simply how things were done, he was taught it was right to own slaves as a child, his friends and family reinforced that, and both governments he worked under endorced it, slavery was an immutable fact of life and he didnt really have any reason to even consider ending it. Most people dont really think too hard about things unless there is something that triggers the persons attention, and at the time so few people protested slavery washington probably went through his life never encountering a "trigger".