Because McCain is a natural citizen, born in U.S territory to a father who was working in Panama on government business (as a diplomat? I think), living on a U.S. military base.
If it were that simple, there would never have been any basis for controversy, no need for a "Sense Of The Senate," and certainly no need for a special resolution redefining "natural born citizen" from Barack Obama. I'm recalling that he wasn't born in US Territory, he was born in a hospital outside the Zone.