Why do most Indian restaurants in the U.S. serve Punjabi food?
A lot of the early migrants from India to the US and UK were either Punjabis or Gujarathis. I think the Punjabi case is true for the UK and you will find a lot of Tandoori type restaurants there. Seems like that may be one of the causes. The other reason could be most of the other cuisines are vegetarian in nature and may not work in the US. Gujaratis are vegetarian and so are quite a lot of the South Indians.