Indian regional cuisines differ most fundamentally in which grain sits at the center of the plate. That division follows rainfall and soil rather than culture, and everything else arranges itself around it.
Water availability decides the crop
Rice needs standing water through much of its growth and thrives where monsoon rainfall is heavy and reliable, which describes the eastern plains, the coasts and much of the south.
Wheat is a cooler-season crop that grows on comparatively less water, suiting the drier northern and western plains. Where irrigation later expanded, wheat spread with it.
The dry interior of the Deccan supported neither well before irrigation, and its traditional staples were sorghum and millets, which tolerate low and unreliable rainfall better than either.
The grain decides the bread
Wheat's gluten allows dough to be kneaded and rolled thin, which is why the wheat belt eats flatbreads made fresh at each meal and treats them as the plate itself.
Rice cannot be rolled into bread, so rice regions either serve the grain whole or grind it into batters that are steamed or griddled. The technique is dictated by the protein the grain lacks.
The staple decides what goes with it
A dry flatbread needs something moist to carry it, so wheat regions developed thicker gravies and drier vegetable preparations that sit alongside rather than soak in.
Rice needs a liquid to bind it into a mouthful, which favors thinner, more pourable preparations. Sambar, rasam and coconut-based gravies are all designed to be mixed into rice by hand.
Souring agents follow the same logic, since a thin poured gravy carries acid more effectively than a thick one clinging to bread.
Fat and dairy track the same lines
Cattle-raising suits the drier north and west, so dairy fat and dairy products feature heavily in those cuisines. Ghee, yogurt and milk-based preparations concentrate there.
Coastal and southern regions grow coconut abundantly, and coconut oil and coconut flesh take the roles dairy plays elsewhere, providing both fat and body.
The division has softened without disappearing
National distribution, migration and irrigation mean wheat is eaten in the south and rice in the north routinely. Restaurant menus in both regions carry the other's staples.
Everyday home cooking has shifted less. The grain a household defaults to when nothing special is planned still tends to follow the older agricultural map more closely than restaurant food does.