Convenience.
It's not just that it has wording designed originally with slavery(former slaves) in mind, but it's also due to the fact that [at least in my experience] the idea of applying originalism to the 14th amendment is one of the most unpopular ideas in the United States.
Progressives most certainly don't want to be all originalist with the 14th amendment.
Here is a clue. Most conservatives don't want to be all about the originalism with the 14th either. I don't see it. Typically, originalism is only supposed to apply to the first authoring of the Constitution, and perhaps by extension encapsulated by the inclusion of those amendments comprising the Bill of Rights. Originalism for some odd reason ends there.
Originalism could have put an end to a lot of our problems decades ago. It's not hard to go reading the debate notes from when they created the 14th amendment. Guess how many times in 1865 they discussed gay marriage, for example?
I'm getting a visual of a broken bone that hasn't been set correctly.
I want to be originalist to the original constitution. You know, the one that doesn't allow occupation armies to "vote" for the people of states, but one that lets the citizens of those states vote for themselves.
In such an originalist interpretation, the 14th amendment doesn't get ratified, because all the citizens of those occupied Southern states would not vote for a legislature that would ratify it.
Allowing the post civil war amendments to proceed as they did was a violation of the original intent of the US Constitution, which Ironically is the very thing they claimed to want to impose on those Southern states.
I guess violating the constitution is okay when they do it.
But the 14th amendment could have been written better, and much deliberate misunderstanding would have been avoided had they done so.