Agree it's easy but who even knows about SOU unless SOU start advertising the airport to Londoners (or anyone within the LHR catchment). SEN is carving out a niche as it is calling itself London Southend. SOU can't do that; they're caught between a rock and a hard place. So apart from a handful of seasonally-well traveled business people who know about SOU's convenience it will never appeal on a mass scale for the reasons mentioned in posts above.
Very disappointing because for a catchment area of our size we need an airport with similar routes as Newcastle, Bristol, Liverpool, East Midlands, etc etc
Unless of course:
Gov removes APD from regional airports thus making fares cheaper and enlarging appeal
Ryanair starts flights from SOU