Nepal Disaster: Why So Many People Were in the Wrong Valley?

Every time this happens, the same question arrives within a day. Why were they there?

This video answers it, and the answer is not what the question expects.
Take a satellite image of Nepal and look for the flat ground. Not gentle slopes. Genuinely flat, buildable, farmable land, where you could put a house without cutting a terrace into a mountainside first.
There is almost none of it.

The country climbs from lowland plain to the highest mountains on Earth across a distance that is, in places, under two hundred kilometres. One of the steepest transitions anywhere on the planet. And in that terrain there is exactly one kind of place where the ground is level, water is available, a road can be built and crops will grow.
The valley floor.

Here is the part that matters. The valley floor is flat because a river made it flat. Water, ice and debris moved through that space for thousands of years, carved it, widened it and left the sediment that makes it level and fertile.
The place that is habitable is habitable because the mountain drains through it.