WAAS is only an augmentation system. A other words a precision addition if you like. You need FIRST to have the GNSS set up in place ( like PBN routes and APP, etc.) drawn up, certified and published . Then you can augment.
Last to have PBN you need procedures, ( not too expensive to make ) and the certified equipment on board. That one can be quite expensive if we talk retrofit.
For Nepal, you have no procedures and no equipment. You cannot do PBN/GNSS with a handeld Garmin taped to the dashboard . Most of the airlines ( with perhaps the exception of Buddah) have not really money for this. I am not even talking of the extra training that would be required .
Maybe when the tourists will stop boarding and start asking questions, then perhaps, we will see this implemented.
But as long as ticket price is low and the tourists consider jumping in a old Do228 in marginal weather in mountains part of the adventure, the need to change is relative.