First you have to consider the context in which wars are initiated. Especially in modern times, a serious invasion doesn't happen for no reason. Most wars are expensive and only worth embarking on if you believe you're going to get something valuable — oil, minerals, ports or other access for trade, etc.

Nepal is an isolated country reliant on agriculture and its infrastructure modernized fairly recently. Its location and the terrain make for what would be a difficult war. Furthermore, during the colonization period, it was more valuable as a buffer between British India and China. Having a buffer state between two powers can itself be helpful because it relieves pressure and tension.

It's also worth pointing out that there was the 1814-16 Anglo-Nepalese War between the British East India Company and the then-Kingdom of Gorkha. Nepal ended up losing some territory as a result of the war. But as mentioned above, the British didn't have any serious interest in trying to conquer the whole state because it would have needlessly antagonized China. Nepal wasn't so valuable to make conquering it worth the increased tension.