Why were African Americans denied the rights they had been granted with the passage of the Reconstruction Amendments?

The Reconstruction Amendments were the Thirteenth (Freed Slaves), Fourteenth (Citizenship rights and equal protection under all laws), and Fifteenth (Voting Rights).

The promise of these amendments was eroded by state laws and federal court decisions throughout the late 19th century. In 1876 and beyond, some states passed Jim Crow laws that limited the rights of African-Americans. Important Supreme Court decisions that undermined these amendments were the Slaughter-House Cases in 1873, which prevented rights guaranteed under the Fourteenth Amendment's privileges or immunities clause from being extended to rights under state law; and Plessy v. Ferguson in 1896 which originated the phrase "separate but equal" and gave federal approval to Jim Crow laws. The full benefits of the Thirteenth, Fourteenth, and Fifteenth amendments were not recognized until the Supreme Court decision in Brown v. Board of Education in 1954 and laws such as the Civil Rights Act of 1964 and the Voting Rights Act of 1965.