Question

why are plains very thickly populated compared to hills

Answer 1

Plains are the rich source of minerals. They are drained by the rivers flowing from the rivers of Himalayas. They are stable and the topography of the place is suitable for living.

Answer 2

Plains are more fertile land with good irrigation facilities . So it is the best source for agriculture. Plains are rich in minerals . The most important livelihood of the people is agriculture . So people used to live in plains rather than the hills.Hence plains  are thickly populated.