Bus stations in Nepal (also Laos, Myanmar and some other Asian countries) are mostly outside of the town center.
In a sense it is good, since theoretically it can decrease the pollution concentration inside the city, practically polluting the outskirts instead.
On the other hand, the "bus stations" are usually dirty and lacking of infrastructure, transport to the center related scams are galore.
Probably some agreements under the table between transport associations and municipalities are the base of this decision.
It can always be another, ver smart and practical reason which I am not familiar with. You never know that with Asia.