Why were most African American still unable to vote even after the fifteenth amendment was adopted?

Most African Americans were still unable to vote even after the fifteenth amendment was adopted because Black Codes and the Grandfather Clause were implemented in the southern states, where it made virtually impossible for African-Americans to vote. This was because it stated that people who had to vote had to be literate, and many freed slaves did not know how to read and to write and the Grandfather Clause stated that people could only vote if their grandfather owned land, but this was impossible for African-Americans, as their grandfathers were slaves and were not allowed to own land.