An Indian street vendor typically sells one dish and its close variants rather than a menu. The narrowness is a deliberate structure that makes a very small business viable.
Equipment is the binding constraint
A cart has room for one primary cooking surface and one fuel supply. A griddle, a fryer and a steamer cannot coexist, so the menu is limited to what one setup can produce.
Choosing a single dish means the equipment is exactly matched to it rather than compromised across several. The vessel, the fuel level and the layout are all tuned to one process.
Adding a second dish with different equipment needs would halve the space available for the first and slow both, which is why carts split rather than expand.
Prep scales only with repetition
A vendor doing morning prep for one dish makes one large batch of one thing, which is far more efficient per unit than preparing several small batches of different things.
Ingredient purchasing simplifies in the same way. Buying a large quantity of a few items gets better prices and reduces the risk of unsold stock spoiling before it can be used.
Waste falls further because leftovers from one day feed into the next day's batch of the same dish, which is only possible when everything prepared belongs to a single preparation.
Speed comes from doing one motion repeatedly
A vendor making the same item hundreds of times a day performs each step without deliberation, and the resulting throughput is what makes low margins per plate add up.
Switching between different preparations interrupts that rhythm, requires reorganizing the working area, and slows the queue at exactly the moment volume matters most.
Reputation attaches to the single item
A cart known for one thing is competing on that one thing, and customers travel for a vendor with a reputation. A general menu gives nothing specific to be known for.
That reputational concentration also disciplines quality, since a single bad batch affects everything the vendor sells rather than one line among many.
Clusters do the work a menu would
Street food areas gather many single-item vendors in one place, so a customer walking a few steps assembles a varied meal from specialists.
The cluster provides the variety while each operator keeps the efficiency of specialization. American food truck lots and pods work on the same principle for the same reasons.