They didn’t. In 1790 after the constitution was adopted, the first congress passed a naturalization act that defined some specific cases to be treated as a “Natural Born Citizen”. However in 1795 they turned around and repealed the 1790 statue and replaced it with nearly identical language except the wording “Natural Born Citizen” was changed to simply “Citizen”.
Since it was the first congress, it’s thought that the 1790 statute might play a role in defining what the original intent of the term “Natural Born” was. But since they repealed that statute and changed the language just 5 years later, I think the repeal would also play a role. Still the most logical explanation is that the term refers to the “Natural Born” as defined in “The Law of Nations” which they would have been familiar with.
The English translations of The Law of Nations available to the drafters did not include the term “natural born.” Blackstone had “natural born subject” and the law of England at the time defined “natural born subject” as born on English soil (with some exceptions for the children of diplomats of invaders) or statutorily a subject at birth (so children of the English born oversses would be “natiral born subjects”). A fair reading of “natural born citizen” in the Constitution could be analogous to “natural born subject”, with either born on U.S. soil or statutorily a citizen at birth. The 1790 statute indicated that Congress was thinking this way. 8 of the 11 of the Committee of 11 that imported “natural born citizen” into the Constitution were members of the 1790 Congress and there is no record of any of the 8 objecting to the use of “natural born” in the 1790 statute.
It defined a single case, that of a child of citizens who was born overseas. Such a child would be a natural-born citizen. It did not mention the status of children born in the U.S.