Because it's in the future...

I shamelessly stole this joke from this guy:

A new Iranian ambassador to the U.N. had just arrived in Now York. President Bush was also at the U.N. giving a speech. They met at a reception afterwards and, trying to be diplomatic, the ambassador introduced himself and began to talk about his impressions of America. "I really love your show Star Trek," he said. "It has a white man, an asian, and a black woman all working together to explore the universe. It's wonderful. But tell me Mr. President, why are there no Iranians on Star Trek?"

Mr. Bush looked the ambassador in the eye and said, "because it's in the future."