Why did the framers of the Constitution require that a President be a natural born Citizen?

Because someone who was born in Another Country, even though they have become an American citizen, might still have sufficient regard or affection for their mother country to put it's interests above ours. Michael Montagne

And also, because the Founding Fathers wrote this into the Constitution. The people have the power to change this requirement, and any other part of the Constitution. This power belongs to the people alone. Changes to the Constitution are not made by the President, or Congress, or even the Supreme Court.