Question

why are most of the ppeninsular rivers are seasonal

Answer 1

Because they depend on rainfall.
The sources of the peninsular rivers lie in plateaus and low hills devoid of snow.
These rivers have smaller courses and small basins.

Answer 2

Peninsular rivers are seasonal as they are rain-fed rivers i.e. they are filled up only by rain water. they do not have a water source which can provide them water like the rivers of the northern plains which are fed by the glaciers of Himalayas.