Two unrelated Indian restaurants in different American states will often list nearly the same dishes in nearly the same order. The similarity is a rational response to shared constraints.
Diner expectation sets the required list
A restaurant that omits the dishes American diners already know will spend every service explaining itself, and explanation is expensive during a dinner rush.
So the familiar items appear whether or not the kitchen considers them representative, functioning as an entry point rather than as the chef's argument.
Anything unfamiliar then has to earn shelf space beside them, which limits how much of a menu can be devoted to it.
Shared prepared bases keep service possible
Kitchens build many finished dishes from a small number of gravies made in the morning, which is how a long menu is served from a small line.
A dish requiring its own separate base costs prep time, storage space and waste risk, so it survives only if it sells well.
The economics push every menu toward the same compact set of bases, and menus that share bases end up sharing dishes.
Supply chains narrow the options further
Restaurants order through distributors that stock what many customers buy. Ingredients outside that catalog require a separate supplier and a smaller, less reliable order.
That favors dishes built from widely distributed items, and disfavors anything depending on a regional ingredient or a short season.
Where a large South Asian grocery wholesale operation exists nearby, menus visibly widen, which is why cities with dense communities support more varied restaurants.
Staffing shapes what can be cooked nightly
A kitchen can only produce what its cooks can make consistently under pressure, and hiring pools favor cooks trained in the same common repertoire.
Dishes needing a specialist, such as a tandoor operator or someone practiced at fermented batters, appear only where that role is staffed.
This is why regional restaurants often start as small operations run by the person who cooks, where the specialist and the owner are the same individual.
Where the divergence is happening
Menus widen through specialization rather than expansion, with small places building around one region, one meal or one format instead of adding dishes.
A narrow menu lets a kitchen carry ingredients that a broad menu could not justify, which inverts the usual logic and makes unusual dishes affordable.
For a diner, the reliable signal is a short menu that omits the expected list, since that omission is a deliberate and costly decision.