Because he is the ultimate game theorist/strategist. His foreign policy maneuvering would run circles around Russia/China/EU/Mideast.
During election campaigns, he would probably come across as a cold fish though. He needs to learn to kiss and hold babies.
As for some of the other choices, I can't imagine President Julian Sark. America's loyalties would be extremely flexible. Anyway, neither he nor Sydney are old enough. Have to be 35 for President, I think. I imagine President Arvin Sloane would eventually be impeached for abusing his powers of office in pursuit of Rambaldi.