You poor thing, you just posted the very sentence which shows the founders differentiated mere citizen from natural born citizen ... if there were no difference, why address two separate types of citizen while not yet addressing naturalization? But keep on trying to shove your square peg barry bassturd into the round hole. It is instructive for readers.
Because there was one generation that wasn't.
The very people who wrote the Constitution itself.
They were not born in the US (because it didn't exist) and so they were ALL naturalized citizens. Now they did not want a naturalized citizen to hold the presidency, but they also did not want to elude everybody over the age of zero, so they wrote an exception that IF you were a naturalized citizen at the time of the signing of the Constitution, you could still be president.
That condition expired when the last person born before on on September 17, 1787 also expired.