Answer:
It's because of Nepal's topology.
Step-by-step explanation:
Around 75% of Nepal is covered by mountains. In terms of transportation, They're considered as the most rugged and difficult terrain to pass through in the world.
This make the use of air transport becomes really integral to reach some areas of Nepal. People usually use road transport to a certain point (usually the big cities) and continue their journey to other parts of the country using small airplane.