Question

why plains are thickly populated compared to mountains

Answer 1

Plains are thickly populated because
1) The soil is very fertile and as they are flat agriculture is possible. In mountains there is constant erosion so fertile soil is deposited in the plains.
2) Wherever rivers were there, we have always found a civilisation. People tend to settle in places where sources of water is easily available.
3) Transport in the plains is easy and also vast

These are the reasons why plains are thickly populated as compared to mountains

Answer 2

In the plains, it's easy for transport and communication for various industries to have its raw materials and also the land is suitable for growing crops whereas in the mountainous region the land is not suitable for agriculture and also transportation system is also not well-developed . It is why plains are thickly populated as compared to mountains.