Until 1956 there was no motorable road into the Kathmandu Valley. That single sentence is the whole explanation, and it is more extraordinary than it first sounds, because Kathmandu in the 1930s and 1940s was a capital city with a palace quarter, a court, imported European furniture and a ruling family with a serious appetite for motor cars.
The transport chain stopped well short. Goods came by rail to the Nepali border and on the short Nepal Government Railway to Amlekhganj, and a road ran on to Bhimphedi at the foot of the hills. Beyond Bhimphedi there was a trail, and the trail crossed the Mahabharat range and then the Chitlang ridge before descending to Thankot on the valley floor. Anything that was going to reach Kathmandu crossed that stretch on a human back or an animal, and there was no exception made for machinery.
So Nepal arrived at the situation that produced the photographs: cars in Kathmandu, driving on a small network of streets inside the valley, that had physically been carried in over a mountain range by men on foot. There was a short ropeway carrying general freight over part of the route from the 1920s, but the cars went the old way.