Nepal is a land locked nation with very hilly terrain.
I have reasons to believe that logistics costs may be the prime reasons for such an expensive solution.
More surprising is that its neighbours ( China and India) with huge surplus minting capacity could not underbid distant mints like those of Poland and UK. Interestingly, China mint just happened to deliver 5 million pieces of 20 Rupees commemorative coins to Sri Lanka ( against international competition) only in the month of February 2021.