Because at the time, the American army was still segregated, and African Americans were discriminated in the army, even if they provided the same service for the country during the war against Germany and Japan.
Fortunately for African Americans, the army was desegregated after the war, and in the following decades, the Civil Rights Movement would lead to desegregation in most public and private places across the country, especially in the South.
I believe it was because of things like segregation and "separate but equal." Even after obtaining citizenship they were still discriminated against by most.
