Because the definition of Natural Born Citizen that requires both parents to be U.S. citizens is nothing more than a Birther delusion and found nowhere in the Constitution.
Don't like my answer? Take it up with Chief Justice Roberts. He was the one who administered the Oath of Office to Obama knowing full well his parentage.
And exactly how did Justice Roberts know full well Obama's place of birth? What was he privy to that the rest of the nation wasn't? Anyway, it isn't the Supreme Court's job to be a complainent about anyone's "natural born citizenship". It is their job to either accept and rule on, or deny the appeal if it becomes an issue that reaches their Court through the appeals process.