Easy did a 3x daily A319 some years ago, which I used from time to time and it was always well filled. Stansted is nowhere near Kings Cross train station, which unlike STN is a pig to reach by car and worse to get parked there. This is true for many places around the M25. Although I am nearer to Kings Cross as the crow flies, it is far quicker to get to Stansted.
This is exactly why we need domestic flights as well as rail services. Rail often wins on city centre-city centre, but elsewhere flying can be a more convenient, and often cheaper, alternative. This is especially true in the London courbation, where an onward journey from a central London terminus can be over 2 hours.