Why did people say the 15th amendment didn't do enough?

The Court declared that the Fifteenth Amendment "commands that the right to vote shall not be denied or abridged on account of race or color, and it gives Congress the power to enforce that command. The Amendment is not designed to punish for the past; its purpose is to ensure a better future."

Explanation:

it refers to race, i am pretty sure this was a popular term in nazi germany or something.