It's quite simple... most flights are scheduled to be back in STN for the night by midnight, yet a good number are arriving significantly later. Most Swissport staff will finish their shifts so there's not enough staff to cope with the additional, unscheduled demand post 00:00 / 01:00. Every airport, handling agents etc. will all inadvertently have problems with resource planning when the airlines fly significantly off schedule. Of course, it's not as simple as an airport having H24 status.