The best street food stalls in most cities sell one dish and nothing else. The narrowness is an operating decision that produces both the quality and the price.
Space determines the menu before anything else
A cart or a stall front offers a working area measured in a couple of square metres, with room for one cooking surface and a limited store of ingredients.
Every additional dish requires its own preparation space, its own utensils and its own holding area, none of which exist on a cart.
The menu therefore contracts to what a single set of equipment can produce continuously, which is usually one dish with two or three variations.
Turnover keeps ingredients fresh
A stall selling one item moves its entire stock every day, so nothing is held overnight and nothing is bought speculatively.
A varied menu forces the same operator to hold small quantities of many ingredients, each turning over slowly, which raises both waste and risk.
This is why the single-dish stall is frequently the safer choice as well as the better one, since high turnover is the most reliable indicator of freshness available to a visitor.
Repetition produces consistency
A cook making the same preparation several hundred times a day develops timing and judgement that cannot be reached by making it occasionally.
Adjustments to heat, seasoning and resting time become automatic, and the variation between the first serving and the last narrows considerably.
Customers reward that consistency by returning, which sustains the volume that made the consistency possible in the first place.
Supply chains form around the specialisation
A stall buying one ingredient in quantity every day becomes a reliable customer for a specific supplier, and receives better material and better prices as a result.
In many cities these relationships run for decades and involve a particular farm, mill or fishing boat rather than a market in general.
Diversifying the menu breaks that relationship into several smaller ones, each too small to command attention from anyone.
Reading a street for the good stalls
Queues of local customers at inconvenient times are the clearest signal, since visitors cluster at predictable hours and residents do not.
A short menu, visible cooking and a stall that runs out and closes rather than continuing into the evening all point the same way.
Stalls offering a broad menu covering several regional cuisines are usually assembling components prepared elsewhere, which is a different business with different economics.