Evidently, they are not making money on those routes. WN does not hold on to routes for "the glory" or the competition. If the route does not make money, they don't fly it. What a concept! If you work for another airline, I wouldn't crow about "beating" WN at PHL. Aside from the fact that PHL isn't the prize of the century, remember, this will free up equipment and crews to fly other routes. You may find competition you didn't want on another route that you thought was exclusively yours.
WN is fairly new to the Northeast. They will try a lot of different route/city combinations. The ones that work, they'll keep. The ones that don't, they'll drop.