I think the biggest reason by far is
wages.
The runway and the buildings sit there quite happily all night. The cost of heating the tower and an office downstairs is not much, similarly the cost of runway lights and if you want to be seriously forward thinking you can have PCL on those
GA airfields everywhere, with instrument approaches, struggle under the weight of ATC salaries, and the fire crew salaries. The former one cannot do anything much about - it is a direct toss-up between salaries, and operating hours. The latter one can be trimmed considerably by e.g. not allowing AOC operations, but then you lose some juicy bizjet traffic (which may or may not matter). It's very tricky.