The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Leroy Johnson was the first black elected since 1906 because he had a good reputation among the people in Georgia for his dedicated work to support the civil rights of African American people in the 1960s, in the state of Georgia when he was a lawyer. He had a good public image and reputation.
Since 1907, when William Rogers had been elected Senator, no other African American had been elected Senator in the state of Georgia. So the arrival of Johnson to the Senate represented a major accomplishment for him and the African American people that lived in the state.