.a lot of people drive to airports,rather than use public transport
And that is Southend's problem. 18.5 miles from any motorway and if Joe Public is coming from London there is no motorway but just A roads.
Southend's appeal will be it's railway station with easy access to London and the terminal. All that is missing is a 24 hour rail service which the airport operator should be pushing for as the railway will not be an option for the morning wave of passengers.