Why is it forbidden in Bhutan to climb mountains with an elevation higher than 6000 meters?

Since 1994 climbing of mountains in Bhutan higher than 6,000 metres has been prohibited out of respect for local spiritual beliefs, and since 2003 mountaineering has been forbidden completely.

That is a bit curious, because Thimpthu, the nations capital city, is at about 8000 ft, so just being in that city is a species of mountaineering. The Thimphu airport (Paro) is so treacherous that only eight pilots in all the world are qualified to land there.