People who run for office are usually lawyers, and there is a definite ethnic divide in professions. Moreover, immigrants usually don't come to be lawyers, since being good at law requires a mastery of the language and the nuances of the local culture, and native born citizens have a natural advantage.
Their cultural connection is much stronger to South Asia than to the Arab world. Also, many Sikhs do not not wear turbans, so to you or anyone else, they would just appear to be "Indians".
Remember when the Hillary Clinton campaign released a photo of Barack Obama wearing a (Somalian) turban? http://www.theguardian.com/world/2008/feb/26/barackobama.use...
I didn't remember that photo from the 08 election (or even see it in the first place). It makes the skull duggery of House of Cards seem every so slightly less fictional!