Because they were English-trained lawyers and used English legal terminology. The Constitution is full of phrases that had a technical legal meaning under English law, which was widely understood at the time ("Duties, Imposts and Excises"; "Letters of Marque and Reprisal"; "Bill of Attainder"; "Cases in Law and Equity"; "Corruption of Blood"; etc., etc.-- I could give you 50 examples).
English law recognized only two kinds of subjects-- Natural Born and Naturalized. The Constitution used that terminology.
The Constitution did not.
Firstly, in the 1700's, a subject of a crown could never be the same as a citizen of a (Constitutional) republic. That's self evident for obvious reasons.
Secondly, the Constitution speaks of two kinds of citizens:
1. A "Citizen", as being an eligibility requirement to be a Senator or a Rep.
2. A "Natural Born Citizen", as being an eligibility requirement to be POTUS.
Clearly, the framers make a distinction between a "Citizen" and a "Natural Born Citizen" as far as eligibility requirements to hold federal government office.
All Natural Born Citizens are Citizens, but not all Citizens are "Natural Born" Citizens.
No. The term Natural Born Citizen is used only once in the Constitution, in connection with the President. It has no relevence to the rest of the Citizenry. Why did they invent this special term for the Presidential qualification only?
They already had equivalents for “Born in the USA”= Citizen or Native, and “Naturalized”
What was different about a “Natural Born Citizen”?