My research finds that the reasons for menu sameness include some of humanities biggest influences: religion, power, happenstance, and economics. More specifically, the concern in Hinduism about food purity, the development of cuisines, the turmoil after the 1947 partition and the risky economics of the restaurant business.
In A Taste of India, Madhur Jaffrey writes
India has had no long tradition of fine public dining such as exists in France and Japan. Upper-class Hindus, who rarely crossed the 'seven seas' for fear of losing caste and whose meals had to be cooked and served by freshly bathed Brahmins, could scarcely be expected to dine in public places where the food had been prepared and touched by God knows who. Even in my family, where we were quite liberal, I never took a sip from my sister's glass or a bite from her apple. Any food eaten by someone else was consider 'unclean' or jhosta.
So how did Mughal cuisine become the dominant cooking style in American restaurants? In The Oxford Encyclopedia of Food and Drink in America (Edited by Andrew F. Smith), the entry on Indian-American Food has a theory.
The authors propose that there are three primary venues where development of cuisine occurs: imperial courts, marketplaces, and households. The courts are where the leaders display their power and wealth. For food, that means a rich and complex cuisine prepared by formally trained chefs who keep good records for their successors. (of course, there were also many recipes that were closely guarded "state secrets" which were probably passed down orally)
The food of the marketplace is created for the workers of the marketplace, travelers, shopkeepers, and shoppers. The food is designed for simplicity, convenience, and low cost. The entry also says that the marketplace was the birthplace of many foods with long shelf life, like pickles.
The home has the least elaborate cuisine because cost is important and the needs of economical nourishment generally outweigh the need to impress. Many techniques are passed orally from one generation to another.
Thus, the complex and rich food of the courts was the obvious choice for a restaurant menu.
Another possible force that made Mughal food the standard restaurant fare is the chaos brought on by the division of India in 1947. Alan Davidson, in the The Oxford Companion to Food writes
However, so far as the outside world is concerned, the tremendous variety in Indian food, whether brought about by geographical and climatic differences (wheat and breads in the north, use of coconut in the south) or arising from dietary laws (no pork for Muslims, no onions for Jains) or from the caste system, or other causes, has been obscured to a very large extend by a coincidental factor. Most of the Indians who operate or cook in restaurants, inside or outside India, are from Punjab....
...The prime factors were the lack of any restaurant tradition in India, and the inhibitions which prevented members of various castes and religious groups from becoming professional cooks. Thus, in the upheavals which followed the division of the subcontinent into India and the and the two Pakistans in 1947, it was displaced Punjabies (numerous, eager to work, and relatively free of inhibitions) who could most easily become entrepreneurs and operators in the restaurant business. They took on these roles, and it was natural that they should subsequently staff the catering colleges set up to ensure part of the necessary infrastructure for tourism....(the wide popularity of Tandoor cookery is, incidentally, one of a number of things for which Punjabis have been responsible--the villages of Punjab had communal open-air tandoors where housewives would bring their dough to be rolled into rotis and baked by the tandooriya).
South Indian restaurants (like Udupi Palace) are popping up around the country (and having great success, with lines out the door on weekends in some locations), places like Ajanta in Berkeley, CA rotate regional dishes onto the menu, and various places in New York City are serving regional dishes and experimenting with fusion, but the Mughal cuisine still dominates, especially outside of areas without large Indian populations. (some lists of Indian restaurants are here and here)
To sum up:
- The centuries of the Mughal reign led to the creation of a complex and splendid cuisine
- Home-style or street food is too mundane for a restaurant (but that's what people used to say about Italian home-style cooking too, and now some of the hottest Italian places are getting back to simplicity.
- India did not have a fine restaurant tradition until after independence
- The restaurant business is financially challenging, especially for niche markets like Indian food. And most people who start restaurants do not have a financial buffer that would allow for experimentation. They must use tried and true concepts to avoid financial failure.