Could the reason perhaps be that SOU's physical characteristics render it unviable for major European airlines? It is currently viable for an airline operating regional turboprop equipment such as Flybe, hence why they have a large base there.
To me the masterplan looks designed to make SOU more viable for short/medium haul jet operators in a way that it isn't currently. The owners have to do this really, as with the current performance and airfield constraints the airport is trapped in a segment of the market that appears to be becoming less viable as the years go by, so I think they're attempting to future-proof the airport by equipping it for the next level of operator up from the regional turboprops it currently caters for.
For a recent example of this, see Southend. In my view once the infrastructure is in place operators will start to arrive.