Not all carriers can fill widebodies to the secondary cities. PER and some ADL flights from SE Asia can be rotated with the 737/A320 where range permits and depending on season (some may have NW as high season and NS as low season for example)Isn’t one of the major attractions of SQ (etc) the fact that they don’t generally fly single aisle jets on long routes.
The MH circus with BNE initially filing for capacity equivalent to the 737-8 MAX and withdrawing and back and forth before eventually announcing they"re restarting with the A333 is another example.