Norwegian moving all flights out was inevitable. I do believe that one day they will completely return though, unless charges at LGW are significantly lower long term. Considering the majority of their pax are inbound, it makes no real sense for them to serve an airport that is much further south than STN, and that offers few benefits to the pax in terms of transferring to London.
Blue Air is a surprise though, and you could say crazy. They are moving from an airport where they have no direct competition and good loads (to Bucharest) to an airport where there is an existing service from an established operator (Wizz). Surely if yields are the problem, moving to LTN can only make that worse.