Why does the us government only address certain equal protection cases?

"The Fourteenth Amendment was implemented in 1868, a short time after the American Civil War. It preceded the Thirteenth Amendment which abolished slavery, leading many former Confederate states to adopt Black Codes after the Civil War.To combat the list of Black Codes enacted in Southern states, Congress imposed the Civil Rights Act of 1866.

This Act was a direct effect of the U.S. Supreme Court decision in the Dred Scott v. Sanford case. The law required that all citizen regardless of race and color have the equal benefits of all laws, as enjoyed by white citizens. The doubts that arose with the law under the Constitution that was in existence then lead Congress to implement changes to the Constitution, which became known as the Equal Protection Clause of the Fourteenth Amendment.  

In order to ensure the fair practice of the Equal Protection Clause, the U.S. Supreme Court decided to apply different tests to the different State classifications and its response to fundamental rights. Usually the Court finds a State classification Constitutional as long as it has a “rational basis” to a “legitimate state purpose”. The U.S. Supreme Court, however, established a firmer sense of analysis to certain cases." -Constitution.laws.com

So basically what I got from that is that the states all have different definitions of 'equal protection'. I think a good comparison would be gun laws: Each state has their own regulations and restrictions on guns, but because of the second amendment guns are legal in all 50 states.