Because of the passing of the 15th Amendment by Congress which allowed for voting regardless of race, people of color, especially African - Americans poured into ballot locations to place their first votes. White Southerners didn't like that, so they turned to the state governments who decided to make all voters pass a literacy test before they could vote. As to avoid violating the Constitution by forcing only African - Americans to take it, it was given to all races, but most who oversaw the voting process allowed some whites to skip the tests and go right on to the ballot boxes.
This was a time when very few African - Americans had an education and could barely read, so the test was already difficult. To add injury to that, the questions were sometimes asked in another language or were questions only answered by someone of high degree. Even those who did get it right were sometimes lied to about their score and if they missed one question, they couldn't retake it and were sent out.
