In truth, there are a lot of factors that have led to the popularity of THIS conspiracy. Obama's name, his father's nationality, the fact that Obama as a child lived in Indonesia, the fact that he was born in Hawaii (outside of the mainland U.S.A.) all helps people who are already inclined to dislike Obama to believe that he is an outsider, a foreigner. However, to answer Sam Quentin's point, if Obama had the last name of Jackson and was born in Alabama, there would still be a conspiracy theory that would be quite popular, it would just be very different. The root of the birther conspiracy is that the people who bought into it really don't want Obama to be POTUS.