It's called affordable safety. Flight out of ATC hours costs less than a flight when the TWR is open. Simple maths. If ASA changed the opening hours then the airlines would reschedule their arrival times to earlier. Same, same for closing times. Business hours means that either a single ATC could operate the TWR, or maybe 3 ATCs over slightly longer hours. Extend further then you probably need a 4th ATC per day.
Airlines don't build their schedules to avoid ATC more a case of people wanting to be places at a certain time which means early/late flights from regional towns/city. REX run an entire airline on that principle.