Good Day!
In this issue they might have some other reason about it.
Based on what I've read in harrotools.com this is illegal by the fact that the business of Nepal is hard to monitor.

In fact, the black money can be taken into another country easily with the help of bitcoin. Hence, drug trafficking, fraud and another criminal activities can be increase with the use of BTC.

In addition to this information, Nepal has its own rules also that until regulations are conceived bitcoin, and other cryptocurrency exchange businesses are illegal. Which is some of the people that wants to against the law is now violated to it.

This country is just only protecting their own safety especially with their own currency.

They're not only the one that do the same things some other country also do the same things, in order to protect their own currency and the money of their co-people.