The abolisionist movement was active in the US for 100 years before the founding of the country. Slavery was outlawed for the first time in the US in 1780. The idea of owning slaves being immoral wasn't novel during the time of Washington. He owned hundreds of slaves for his entire life. They weren't granted freedom until 2 years after his death when Martha freed the ones she inherited from him against his wishes (he stipulated their freedom upon Martha's death, how convenient.)
It might have been immoral, but it was an economic necessity at the time. So was child labor 150 years ago, most families didn‘t have the luxury of sending their kids to school. In 2-300 years who knows what people will think of us for practices we see as necessary or normal today.