Restricted hours has not stopped Easyjet at Southend so I would not get too bogged down on that one. Costs become a major issue if Southampton opened with longer hours. As well as ATC, fire cover would have to be provided plus many other labour costs to keep the airport open.
Also don’t dismiss the rights of locals to have a good nights sleep. They might have bought homes close to an airport but they bought their homes in the knowledge that the airport was closed each night.