Why were the 14th and 15th Amendments added to the Constitution after the 13th Amendment ended slavery?

Because the 13th Amendment didn't grant full citizenship rights to free African Americans.

It was passed by Congress on January 31, 1865. Yet, although former slaves were now freed by the constitution, they didn't have all the rights as all citizens, and some people, especially on South planned to use that.

That is why 14th and 15th Amendments were brought to power in order to make all people free and equal by the Constitution.

The 14th Amendment was adopted on July 9, 1868. It said that all people who were born and naturalized in the US are ought to be granted citizenship. That included recently freed Afro Americans who were not legally or informally considered to be citizens by many Southern states The 15th Amendment was ratified on February 3, 1870. It stated that all the citizens of the US were to be granted a right to vote, despite their "race, color, or previous condition of servitude". However, South still tried to avert and forbid African Americans the right to vote, so finally, in 1965. The Voting Rights Act prohibited further discrimination based on race when it comes to voting rights. Even after that, some states were still to ratify the Amendment in years to come (Maryland in 1973, Kentucky in 1976, Tennessee in 1997)