Answer:
Because the letter argues that black people are not ready to rule themselves.
Step-by-step explanation:
The letter you are asking about was published in 1873. Its content can be sum up as the personal opinion of a white supremacist, someone who believed that black people couldn't rule themselves because they were propense to violence, moral ambiguity, and their lack of education. According to the letter, it was necessary that a new generation of black people would emerge to obtain the rights to rule something, but they had to pass for probation (a test), to see their predisposition. Even so, according to this person, they would fail.