Since the days of many of the named operators flying 737-300/400 aircraft from SOU, there have been changes to the safety criteria around calculating required runway lengths. This has, in broad terms, increased runway requirements by 15% to add a safety buffer. On short runways, that directly impacts on capacity and therefore viability.
A further factor is that several of the issues at SOU are obstacles - trees. Trees grow. Over time, obstacles that were not particularly limiting have now become so. That's why the airport is engaged in a major tree removal project to mitigate these obstacles. A new Type A obstacle chart will then be required after a survey to identify and chart which obstacles have been removed. Only once that official chart is available can airlines then recalculate their performance data to see how big an improvement has been achieved based on a particular aircraft's lifting capability at a particular temperature with wet/dry runway conditions.
It's therefore not really relevant to look back 20 years and say "Airline X did this". So much has changed.