From London, the south of London and also from Essex and the Thames Estuary, it is often an abundantly convenient airport.
Go south or east from Southend and it isn't long before you hit salt water so it's catchemnt area is in 2 directions only.
A part time train service that is even slower than Stansted and 20 miles from the nearest motorway is not going to help the cause either but saying that getting through Southend's terminal is much quicker than most other airports but that will change as passenger numbers increase.
Always thought Stansted was a fine airport but the airlines seem to have issues with the place or it is issues with Ryanair?