Why was Nepal left alone by the European colonalist when every other country where conquered by them?

Answer 1

Because access was so difficult and there were no natural resources to plunder

Answer 2

Because the Gurkhas proved to be too much for them. When the British encountered the Gurkhas, they found that they had more than met their match.

After the hostilities ended, Great Britain began raising regiments of Gurkhas. When I was in Iraq, I saw hundreds of Gurkhas. Most of them are short and compact. A few of them were taller. They were very polite and friendly, and all the Americans liked them. I am glad that they were on our side.

Incidentally, when Iraq invaded Kuwait, the Kuwaiti Army was composed of foreign mercenaries. The mercenaries fled with the exception of the Gurkhas. They stood their ground.