Because the 14th Amend is written in legalese code. Until you look up the definitions of every word in Bouvier's and Black's Law dictionaries you will not comprehend that it ultimately defines a US citizen as an officer, agent, or employee of the Federal Govt, and by extension of any State gov't since all are incorporated under the Federal umbrella. Just substitute "officer, agent, or employee" everywhere it uses the word "citizen" and you will see that they were protecting their employees / agents from any action of due process under state laws during reconstruction. It was written deliberately obscure in order to hide it.