Why did the author know almost nothing about his ancestry?
The most direct answer to this question is that Washington's lack of knowledge about his ancestry is the embodiment of what slave owners wanted. The dehumanization of the slave was accomplished when they did not recognize or understand their past. Slaves were conditioned to view the present as the only relevant condition in their being. It is not as if Washington chose not to know about his ancestry. As part of the condition of slavery, he did not have a choice in the matter.
Slaves were cut off from their past in a universal sense in terms of race or ethnicity and in a particular sense in how they were broken from their families. Washington had little knowledge of his extended family, a reflection of the purpose of slaveowners who broke up families to send to different locations. His lack of a father was another condition of enslavement, in which there was no family unit encouraged. Slave owners wanted slaves to be as alienated as possible, as this made it easier to dehumanize and exploit them for purposes of labor. For Washington, his lack of knowledge about his ancestry is the very essence of what helped make slavery such a dehumanizing institution in American History.