India divides roughly into a wheat-eating north and a rice-eating south and east, and the boundary follows rainfall rather than culture. The consequences run through everything else on the plate.
Water availability drew the line
Rice requires standing water through much of its growing season, which suits high-rainfall regions and river deltas. Wheat grows on far less water and tolerates cooler, drier conditions.
The northern plains and the drier west therefore grew wheat, millet and sorghum, while the coasts, the east and the far south grew rice.
Because grain was bulky and expensive to move before modern transport, what was grown locally was overwhelmingly what was eaten locally.
The grain determines how the meal is eaten
Flatbread is torn and used to pick up food, so it works with dishes that are thick enough to be held by a piece of bread and does not suit thin liquids.
Rice is eaten by mixing, so it accommodates thin, sour and heavily liquid preparations that bread simply could not carry.
This is why southern meals include several thin, distinctly sour dishes served in sequence over rice, while northern meals favour thicker gravies.
Grinding versus soaking
Wheat is milled into flour and mixed into dough, which requires gluten to hold together and a griddle or oven to cook. The equipment and technique follow from that.
Rice is cooked whole, or soaked and ground wet into batters that are then fermented. Neither route resembles the other in method or in equipment.
Kitchens in each region are therefore built differently, with the wet grinder central in one and the griddle and rolling pin central in the other.
Millets occupy their own position
Sorghum, pearl millet and finger millet grow where both rice and wheat struggle, and they were the staple across large dry regions rather than a marginal crop.
They contain no gluten, so their breads are patted out by hand rather than rolled, and they are cooked and eaten immediately because they stiffen quickly.
That fragility is a large part of why they travelled less than wheat breads did, despite being eaten daily across substantial areas.
What changed with distribution
Modern transport and public grain distribution moved rice and wheat far beyond where they grow, so both are now available almost everywhere.
Household habits have shifted accordingly, and many kitchens now cook both, though usually with one still treated as the everyday staple and the other as an occasional alternative.
The dishes have moved less than the grains, since a preparation built around thin sour liquid still works best with rice regardless of what else is in the kitchen.