Late night STN in winter would be popular alright but the airport's winter strategy in recent years has been to stick rigidly to daytime hours to cut costs - hopefully they can finance something different this winter as it would undoubtedly boost pax numbers.
To run a late flight would likely cannibalize the passengers from the day.
Its not going to give you 2 full loads of passengers.
This would also require the Airport to bring in staff or extend the day shift.
Either way it would cost money.
The knife edge that airports run on is very narrow.
Local management may desire a late flight but are no fools, if they can get the same passengers to use the airport during its scheduled opening hours its a far better use of resources.
Remember that the revenue generated by late opening especially in winter must be balanced by the cost of providing the service.