Question

why the plains are more thickly populated than the desert of the mountains?

Answer 1

plains are thickly populated because it is good for agriculture.
soil are fertile
water level is good

Answer 2

Plains are more thickly populated than deserts or mountains because Plains are formed by rivers deposits and thus they are fertile and fit for agriculture.
The topography is promotes proper communication, transportation and other facilities. The climate is also suitable in the plains compared to the extreme climatic conditions in other places.