Answer:
The main reason is slope and high altitude of mountains with low water availability.
Explanation:
- The soil of mountain is present in thin layer with low humidity.
- And due to slope like nature of mountains when it rains, rain water does not absorb in soil while it washes away fertile soil and nutrient with itself.
- While the soil in valleys is fertile because valleys are plains with thick layers of soil having different nutrient types.
- And when it rains, rain water collected and absorbed in plain land making soil fertile.
