Question

Why narmada and tapi don't form deltas

Answer 1

Narmada and tapi are western rivers and Wastern rivers do not form Delta whereas they form Estuaries because, ... Hard Rocks of Western ghat don't allow Western flowing rivers to widen their mouth into sea, so these rivers form Estuaries whereas Eastern rivers flow through broken hills and mountain ranges.

Answer 2

The reason behind the formation of deltas in the east coast lies in how the peninsular plateau is shaped. The plateau slopes steeply towards the west coast and has a gentle slope towards the eastern side. This results into:
short swift streams towards the west coast - due to short course these carry less sediment load and thus do not form deltas. (narmada and tapi are exceptions as they flow in rift valleys )
the east flowing rivers cover large distances and deposit their sediments at the coast forming deltas - eg Krishna-Godavari delta, Kaveri delta,etc.
With regards to Himalayan rivers, they form large extensive deltas irrespective of whether they flow east or west - Ganga-brahamputra delata in east coast and the Indus river delta in the west coast. These rivers have long courses, flow through soft sedimentary plains thus carrying large amounts of sediments forming massive deltas dwarfing those formed by the peninsular rivers.