Why was the Voting Rights Act of 1965 necessary when the 15th Amendment had already given blacks voting rights?

The 15th Amendment was passed, granting African American men the right to vote, on this date in 1870. With discrimination still largely unchecked, violence against blacks at voting polls was rampant. Literacy tests, poll taxes and other voter qualification laws became common. It took nearly a century and the Voting Rights Act of 1965 for the amendment's intention to be achieved throughout the US. Black women in America attained the right to vote along with the rest of America's adult female population in 1920 with the ratification of the 19th Amendment. But, they, too, had to wait till 1965 to actually exercise that right.
 
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