"Because America" is a legitimate requirement for a Presidential aircraft. The President and what he uses is a powerful statement about the strength of US industry. That's why Queen Elizabeth II has a Bentley, and the French President is driven around in a Peugeot or a Citroën.
That makes sense though Bentley and all other UK cars are no longer UK (now owned by companies from other countries). Boeing is a US company but I was talking with someone who was familiar with the USAF tanker selection and competition. He said if a tanker from EADS will provide more US jobs because much of what Boeing has been off shored (though some of this may have reversed after 787 debacles).