Reason #1: Even after the 15th amendment was ratified, some states still found ways to circumvent the Constitution and prevent all African-Americans from voting.
Reason #2: The Supreme Court struck it down in 1915, many states used the "grandfather clause" to keep descendents of slaves out of elections. The clause said you could not vote unless your grandfather had voted -- an impossibility for most people whose ancestors were slaves, African-Americans.
Reason #3: Discrimination practices also prevented African-Americans from voting.
