Of all the 'London' airports, Southend has many advantages for both airline and passenger.
push back to wheels up has got to be the quickest and therefore cheapest - how many tonnes of fuel does it save? just knowing if you are a couple of minutes late you haven't lost your slot and have a 45 minute delay makes planning easier
same goes for passengers, no 40 minute walk to gate 1, no hour long queues for passport control and security. total that up, for both passengers and taxiing, and it probably saves a good hour or so from arrival at airport to being in the air, and the same on return.
rail links to london no worse than luton or stanstead.
biggest drawback is the airlines not offering the destinations.