The obvious reason is that as EVERYONE already knows that water is wet, the framers all knew what “natural born citizen” meant. In their minds, at the time, no need to define it. I wish they had. You should read what I wrote up above. The word "Citizen" effectively defines itself once you look into where the word originated.
The word is not native to English, and especially not in it's current usage. It's current usage indicates that it was adopted from Switzerland where it was known to mean members of a confederated republic.
In the English of the time period (1770s) the word "Citizen" meant someone who lived in a City.
From "A dictionary of the English language. by Samuel Johnson, 1768." 
I have examined several other English dictionaries from that time period. All of them define "Citizen" as members of a city. None of them define it as members of a nation. They also all acknowledge that the word is "French" in origin.
Saw your post while skimming this thread. You say you "examined other dictionaries," but apparently not the
Oxford English Dictionary which is more accurately a history of the usage of words in the English language than it is a dictionary. You should look at my thread:
On Constitutional Eligibility.
While it doesn't consider what the OED has to say about the word citizen, I just looked and the OED broadens the definition specifically for the US. They do give Johnson's definition first, but the second they give the broader US definition with the first usage example from 1538.
ML/NJ